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GH-39031: [Docs] Remove misspelled rule from contrib css (#39032)



### Rationale for this change

The css rule had a typo which means that it hasn't worked.

### What changes are included in this PR?

Fixes the css rule

### Are these changes tested?

### Are there any user-facing changes?

* Closes: #39031

Authored-by: Josh Soref <2119212+jsoref@users.noreply.github.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>