docs: find-unused-docs.sh: fixup directory usage

The recent move of this script from scripts/ to tools/docs/
did not account for the 'cd' directory usage.
Update "cd .." to "cd ../.." to make the script self-correcting.

This also eliminates a shell warning:
./tools/docs/find-unused-docs.sh: line 33: cd: Documentation/: No such file or directory

Fixes: 184414c6a6ca ("docs: move find-unused-docs.sh to tools/docs")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: 184414c6a6ca (docs: move find-unused-docs.sh to tools/docs)
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260102200657.1040234-1-rdunlap@infradead.org>