Fix mis-markup of the -p, --patch option in git-add(1)

An item in a bulletted list in AsciiDoc is followed with two colons,
not just one.

Signed-off-by: Eyvind Bernhardsen <eyvind-git@orakel.ntnu.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -65,7 +65,7 @@ OPTIONS
 	operation to a subset of the working tree. See ``Interactive
 	mode'' for details.
 
--p, \--patch:
+-p, \--patch::
 	Similar to Interactive mode but the initial command loop is
 	bypassed and the 'patch' subcommand is invoked using each of
 	the specified filepatterns before exiting.