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MINOR: [Docs][Python] Fixed a typo in dataset join docs (#41802)



### Rationale for this change

Found a typo in a code example of Python docs.

### What changes are included in this PR?

`ds1.join(ds2, key="id")` should be `ds1.join(ds2, keys="id")`

### Are these changes tested?

No, just docs.

### Are there any user-facing changes?

Yes, but just docs.

Authored-by: Wenbo Li <424026452@qq.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>