docs: vfs: fix typo in struct xattr_handlers

The structure is called struct xattr_handler, singular, not plural.
Fixing the typo also makes it greppable with the whole word matching
flag.

Signed-off-by: Ariel Miculas <amiculas@cisco.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20231027152101.226296-1-amiculas@cisco.com>