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MINOR: [Docs][Python] Add a missing . (#35831)



### Rationale for this change

Fix typo in compute.rst

### What changes are included in this PR?
document update

### Are these changes tested?

### Are there any user-facing changes?
No

Authored-by: Wenbo Hu <huwenbo1988@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>