[CORE][MINOR] Fix obvious error and compiling for Scala 2.12.7

## What changes were proposed in this pull request?

Fix an obvious error.

## How was this patch tested?

Existing tests.

Closes #22577 from sadhen/minor_fix.

Authored-by: Darcy Shen <sadhen@zoho.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

core/src/main/scala/org/apache/spark/status/api/v1/OneApplicationResource.scala

@@ -175,7 +175,7 @@ private[v1] class OneApplicationAttemptResource extends AbstractApplicationResou
   def getAttempt(): ApplicationAttemptInfo = {
     uiRoot.getApplicationInfo(appId)
       .flatMap { app =>
-        app.attempts.filter(_.attemptId == attemptId).headOption
+        app.attempts.find(_.attemptId.contains(attemptId))
       }
       .getOrElse {
         throw new NotFoundException(s"unknown app $appId, attempt $attemptId")