doc/filters: Fix texi syntax

Commit f1efaf837117718e3b4c96336fe9b5f686b1fe30 used parentheses
instead of curly braces in texi syntax, resulting in warnings when
the documentation is generated. Fix that.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>