ls-remote documentation: <refs> argument is optional

Correct SYNOPSIS section.

Signed-off-by: Piotr Krukowiecki <piotr.krukowiecki@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-ls-remote.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git ls-remote' [--heads] [--tags]  [-u <exec> | --upload-pack <exec>]
-	      <repository> <refs>...
+	      <repository> [<refs>...]
 
 DESCRIPTION
 -----------