git-remote documentation: fix synopsis to match description

In the text, the argument of -m is <master> which should be used in the
command synopsis, too.
Signed-off-by: NJörg Sommer <joerg@alea.gnuu.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-remote.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git-remote'
-'git-remote' add [-t <branch>] [-m <branch>] [-f] [--mirror] <name> <url>
+'git-remote' add [-t <branch>] [-m <master>] [-f] [--mirror] <name> <url>
 'git-remote' rm <name>
 'git-remote' show <name>
 'git-remote' prune <name>