Fix 'git var' usage synopsis

The parameter to 'git var' is not optional.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-var.txt

@@ -8,7 +8,7 @@ git-var - Show a git logical variable
 
 SYNOPSIS
 --------
-'git var' [ -l | <variable> ]
+'git var' ( -l | <variable> )
 
 DESCRIPTION
 -----------

builtin-var.c

@@ -6,7 +6,7 @@
 #include "cache.h"
 #include "exec_cmd.h"
 
-static const char var_usage[] = "git var [-l | <variable>]";
+static const char var_usage[] = "git var (-l | <variable>)";
 
 static const char *editor(int flag)
 {