[MINOR][PYTHON] Fix typo in a docsting of RDD.toDF
[MINOR][PYTHON] Fix typo in a docsting of RDD.toDF

### 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 <c14fc9ad304f314fbabaea1b79d1a2154c9b9ddf@gmail.com>
Signed-off-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>

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