Documentation: add: <filepattern>... is optional

<filepattern>... is optional (e.g. when the --all or --update
options are used) so use square brackets in the synopsis.
Signed-off-by: NPaul Bolle <pebolle@tiscali.nl>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -10,7 +10,7 @@ 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] [--] [<filepattern>...]
 
 DESCRIPTION
 -----------