[SPARKR][MINOR] Fix doc for show method

## What changes were proposed in this pull request?

The original doc of `show` put methods for multiple classes together but the text only talks about `SparkDataFrame`. This PR tries to fix this problem.

## How was this patch tested?

Manual test.

Author: Junyang Qian <junyangq@databricks.com>

Closes #14776 from junyangq/SPARK-FixShowDoc.