### What changes were proposed in this pull request?
Fixes typo in docsting of `toDF`
### Why are the changes needed?
The third argument of `toDF` is actually `sampleRatio`.
related discussion: https://github.com/apache/spark/pull/12746#discussion-diff-62704834
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
This patch doesn't affect any logic, so existing tests should cover it.
Closes#29551 from unirt/minor_fix_docs.
Authored-by: unirt <lunirtc@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>