Fix odd markup in --diff-filter documentation

Fix odd markup in --diff-filter documentation

Instead of using the regex-like bracket expression, use grouping to make
it more consistent with other similar places. The brackets now have the
same meaning as in other documentation (i.e., the argument is optional).

Signed-off-by: Štěpán Němec <stepnem@gmail.com>
Mentored-and-Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>