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: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NDarrick J. Wong <djwong@kernel.org>
Reviewed-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NChaitanya Kulkarni <chaitanya.kulkarni@wdc.com>

Documentation/admin-guide/xfs.rst

@@ -536,3 +536,4 @@ The interesting knobs for XFS workqueues are as follows:
   cpumask        CPUs upon which the threads are allowed to run.
   nice           Relative priority of scheduling the threads.  These are the
                  same nice levels that can be applied to userspace processes.
+============     ===========