xfs: Extend table marker on deprecated mount options table

Sphinx reports htmldocs warning on deprecated mount options table:

/home/bagas/repo/linux-kernel/Documentation/admin-guide/xfs.rst:243: WARNING: Malformed table.
Text in column margin in table line 5.

===========================     ================
  Name                          Removal Schedule
===========================     ================
Mounting with V4 filesystem     September 2030
Mounting ascii-ci filesystem    September 2030
ikeep/noikeep                   September 2025
attr2/noattr2                   September 2025
===========================     ================

Extend the table markers to take account of the second name entry
("Mounting ascii-ci filesystem"), which is now the widest and
to fix the above warning.

Fixes: 7ba83850 ("xfs: deprecate the ascii-ci feature")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>