[MINOR] Issue: Change "slice" vs "partition" in exception messages (and code?)

## What changes were proposed in this pull request?

Came across the term "slice" when running some spark scala code. Consequently, a Google search indicated that "slices" and "partitions" refer to the same things; indeed see:

- [This issue](https://issues.apache.org/jira/browse/SPARK-1701)
- [This pull request](https://github.com/apache/spark/pull/2305)
- [This StackOverflow answer](http://stackoverflow.com/questions/23436640/what-is-the-difference-between-an-rdd-partition-and-a-slice) and [this one](http://stackoverflow.com/questions/24269495/what-are-the-differences-between-slices-and-partitions-of-rdds)

Thus this pull request fixes the occurrence of slice I came accross. Nonetheless, [it would appear](https://github.com/apache/spark/search?utf8=%E2%9C%93&q=slice&type=) there are still many references to "slice/slices" - thus I thought I'd raise this Pull Request to address the issue (sorry if this is the wrong place, I'm not too familar with raising apache issues).

## How was this patch tested?

(Not tested locally - only a minor exception message change.)

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: asmith26 <asmith26@users.noreply.github.com>

Closes #17565 from asmith26/master.

core/src/main/scala/org/apache/spark/rdd/ParallelCollectionRDD.scala

@@ -116,7 +116,7 @@ private object ParallelCollectionRDD {
    */
   def slice[T: ClassTag](seq: Seq[T], numSlices: Int): Seq[Seq[T]] = {
     if (numSlices < 1) {
-      throw new IllegalArgumentException("Positive number of slices required")
+      throw new IllegalArgumentException("Positive number of partitions required")
     }
     // Sequences need to be sliced at the same set of index positions for operations
     // like RDD.zip() to behave as expected

examples/src/main/java/org/apache/spark/examples/JavaSparkPi.java

@@ -26,7 +26,7 @@ import java.util.List;
 
 /**
  * Computes an approximation to pi
- * Usage: JavaSparkPi [slices]
+ * Usage: JavaSparkPi [partitions]
  */
 public final class JavaSparkPi {
 

examples/src/main/java/org/apache/spark/examples/JavaTC.java

@@ -32,7 +32,7 @@ import org.apache.spark.sql.SparkSession;
 
 /**
  * Transitive closure on a graph, implemented in Java.
- * Usage: JavaTC [slices]
+ * Usage: JavaTC [partitions]
  */
 public final class JavaTC {
 

examples/src/main/scala/org/apache/spark/examples/BroadcastTest.scala

@@ -21,7 +21,7 @@ package org.apache.spark.examples
 import org.apache.spark.sql.SparkSession
 
 /**
- * Usage: BroadcastTest [slices] [numElem] [blockSize]
+ * Usage: BroadcastTest [partitions] [numElem] [blockSize]
  */
 object BroadcastTest {
   def main(args: Array[String]) {

examples/src/main/scala/org/apache/spark/examples/MultiBroadcastTest.scala

@@ -23,7 +23,7 @@ import org.apache.spark.sql.SparkSession
 
 
 /**
- * Usage: MultiBroadcastTest [slices] [numElem]
+ * Usage: MultiBroadcastTest [partitions] [numElem]
  */
 object MultiBroadcastTest {
   def main(args: Array[String]) {

examples/src/main/scala/org/apache/spark/examples/SparkALS.scala

@@ -100,7 +100,7 @@ object SparkALS {
         ITERATIONS = iters.getOrElse("5").toInt
         slices = slices_.getOrElse("2").toInt
       case _ =>
-        System.err.println("Usage: SparkALS [M] [U] [F] [iters] [slices]")
+        System.err.println("Usage: SparkALS [M] [U] [F] [iters] [partitions]")
         System.exit(1)
     }
 

examples/src/main/scala/org/apache/spark/examples/SparkLR.scala

@@ -28,7 +28,7 @@ import org.apache.spark.sql.SparkSession
 
 /**
  * Logistic regression based classification.
- * Usage: SparkLR [slices]
+ * Usage: SparkLR [partitions]
  *
  * This is an example implementation for learning how to use Spark. For more conventional use,
  * please refer to org.apache.spark.ml.classification.LogisticRegression.