[llvm][doc] Fix typo in documenation

[alekuz01]

Fix for #71941

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[mgabka]

LGTM, however please check what is the process now when working on github issues and reviews via github, in the past when issue was referenced in commit message it was getting closed after merging phabricator patch automatically, not sure if there is anything like that now.

Also I would also suggest to use a different title for the commit message, perhaps:

[NFC] Fix typo in documentation in CMake.rst

and then in the description mention the guthub issue

[alekuz01]

@mgabka thank you for the feedback. should I amend message?