xfs: fix rst syntax error in admin guide

Tables are supposed to have a matching line of "===" to signal the end
of a table.  The rst compiler gets grouchy if it encounters EOF instead,
so fix this warning.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>