git add: Add --ignore-missing to SYNOPSIS

All the git add options were listed in the synopsis until the
--ignore-missing option was added. Change that so that the git add
documentation now has the complete listing.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -10,7 +10,8 @@ SYNOPSIS
 [verse]
 'git add' [-n] [-v] [--force | -f] [--interactive | -i] [--patch | -p]
 	  [--edit | -e] [--all | [--update | -u]] [--intent-to-add | -N]
-	  [--refresh] [--ignore-errors] [--] [<filepattern>...]
+	  [--refresh] [--ignore-errors] [--ignore-missing] [--]
+	  [<filepattern>...]
 
 DESCRIPTION
 -----------