[SPARK-23291][SPARK-23291][R][FOLLOWUP] Update SparkR migration note for

## What changes were proposed in this pull request?

This PR fixes the migration note for SPARK-23291 since it's going to backport to 2.3.1. See the discussion in https://issues.apache.org/jira/browse/SPARK-23291

## How was this patch tested?

N/A

Author: hyukjinkwon <gurwls223@apache.org>

Closes #21249 from HyukjinKwon/SPARK-23291.

docs/sparkr.md

@@ -664,6 +664,6 @@ You can inspect the search path in R with [`search()`](https://stat.ethz.ch/R-ma
  - For `summary`, option for statistics to compute has been added. Its output is changed from that from `describe`.
  - A warning can be raised if versions of SparkR package and the Spark JVM do not match.
 
-## Upgrading to Spark 2.4.0
+## Upgrading to SparkR 2.3.1 and above
 
- - The `start` parameter of `substr` method was wrongly subtracted by one, previously. In other words, the index specified by `start` parameter was considered as 0-base. This can lead to inconsistent substring results and also does not match with the behaviour with `substr` in R. It has been fixed so the `start` parameter of `substr` method is now 1-base, e.g., therefore to get the same result as `substr(df$a, 2, 5)`, it should be changed to `substr(df$a, 1, 4)`.
+ - In SparkR 2.3.0 and earlier, the `start` parameter of `substr` method was wrongly subtracted by one and considered as 0-based. This can lead to inconsistent substring results and also does not match with the behaviour with `substr` in R. In version 2.3.1 and later, it has been fixed so the `start` parameter of `substr` method is now 1-base. As an example, `substr(lit('abcdef'), 2, 4))` would result to `abc` in SparkR 2.3.0, and the result would be `bcd` in SparkR 2.3.1.