AFS: Documentation: fix a few typos in afs.rst

Fix typos (punctuation, grammar, spelling) in afs.rst.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Cc: linux-afs@lists.infradead.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20210117213351.1075-1-rdunlap@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>