Documentation: show-ref <pattern>s are optional

Specifying one or more <pattern> parameters is optional when calling
show-ref, so mark them as such using brackets in the manual.

Signed-off-by: Holger Weiß <holger@zedat.fu-berlin.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show-ref.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 [verse]
 'git show-ref' [-q|--quiet] [--verify] [--head] [-d|--dereference]
 	     [-s|--hash[=<n>]] [--abbrev[=<n>]] [--tags]
-	     [--heads] [--] <pattern>...
+	     [--heads] [--] [<pattern>...]
 'git show-ref' --exclude-existing[=<pattern>] < ref-list
 
 DESCRIPTION