git-merge-index documentation: clarify synopsis

The options following <merge-program> are not -a, --, or <file>...,
but either -a, or -- <file>..., while -- is optional.
Signed-off-by: NGerrit Pape <pape@smarden.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-merge-index.txt

@@ -8,7 +8,7 @@ git-merge-index - Run a merge for files needing merging
 
 SYNOPSIS
 --------
-'git-merge-index' [-o] [-q] <merge-program> (-a | \-- | <file>\*)
+'git-merge-index' [-o] [-q] <merge-program> (-a | [--] <file>\*)
 
 DESCRIPTION
 -----------

merge-index.c

@@ -91,7 +91,7 @@ int main(int argc, char **argv)
 	signal(SIGCHLD, SIG_DFL);
 
 	if (argc < 3)
-		usage("git-merge-index [-o] [-q] <merge-program> (-a | <filename>*)");
+		usage("git-merge-index [-o] [-q] <merge-program> (-a | [--] <filename>*)");
 
 	setup_git_directory();
 	read_cache();