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MINOR: [Docs][Python] Update `python_test.cc` reference (#44622)



### Rationale for this change

Commit 21dbf4ac09583651899232c9a80b3f5d6580a588 moved the location of `python_test.cc`.

### What changes are included in this PR?

PR updates the reference to `python_test.cc`.

### Are these changes tested?

### Are there any user-facing changes?

No.

Authored-by: Emmanuel Ferdman <emmanuelferdman@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>