[SPARK-15973][PYSPARK] Fix GroupedData Documentation
[SPARK-15973][PYSPARK] Fix GroupedData Documentation

*This contribution is my original work and that I license the work to the project under the project's open source license.*

## What changes were proposed in this pull request?

Documentation updates to PySpark's GroupedData

## How was this patch tested?

Manual Tests

Author: Josh Howes <josh.howes@gmail.com>
Author: Josh Howes <josh.howes@maxpoint.com>

Closes #13724 from josh-howes/bugfix/SPARK-15973.

(cherry picked from commit e574c9973ddbef023c066ccd6f771ab01cbf2d88)
Signed-off-by: Reynold Xin <c9d35a384bc579d1c6c8c055f2a2b5c04e11b521@databricks.com>

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