[TRIVIAL][DOCS][STREAMING][SQL] The return type mentioned in the Javadoc is...

[TRIVIAL][DOCS][STREAMING][SQL] The return type mentioned in the Javadoc is incorrect for toJavaRDD, …

## What changes were proposed in this pull request?

Change the return type mentioned in the JavaDoc for `toJavaRDD` / `javaRDD` to match the actual return type & be consistent with the scala rdd return type.

## How was this patch tested?

Docs only change.

Author: Holden Karau <holden@us.ibm.com>

Closes #13954 from holdenk/trivial-streaming-tojavardd-doc-fix.