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GH-38971: [C++] Fix spelling (filesystem) (#38972)



### Rationale for this change

### What changes are included in this PR?

Spelling fixes to cpp/src/arrow/filesystem/

### Are these changes tested?

### Are there any user-facing changes?

* Closes: #38971

Authored-by: Josh Soref <2119212+jsoref@users.noreply.github.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>