submodule: document "sync --recursive"

The "git submodule sync" command supports the --recursive flag, but
the documentation does not mention this.  That flag is useful, for
example when a remote is changed in a submodule of a submodule.
Signed-off-by: NMatthew Chen <charlesmchen@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -20,7 +20,7 @@ SYNOPSIS
 'git submodule' [--quiet] summary [--cached|--files] [(-n|--summary-limit) <n>]
 	      [commit] [--] [<path>...]
 'git submodule' [--quiet] foreach [--recursive] <command>
-'git submodule' [--quiet] sync [--] [<path>...]
+'git submodule' [--quiet] sync [--recursive] [--] [<path>...]
 
 
 DESCRIPTION