docs: fix typo in file index docs#22

Rohan5commit wants to merge 1 commit intoBurntSushi:masterBurntSushi/winapi-util:masterfrom

Rohan5commit:rohan/20260328-file-index-doc-typoRohan5commit/winapi-util:rohan/20260328-file-index-doc-typoCopy head branch name to clipboard