Documentation: remove backslashes in manpage synopses

For some reason, various manual pages have an asterisk escaped
with \ in the synopsis.  Since there is no other asterisk to pair it
with, Asciidoc does not consider this asterisk escapable, so it passes
the backslash through.

Each page either uses [verse] or has only one asterisk, so it
is safe to drop the backslashes (checked with asciidoc 8.5.2).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-merge-index.txt b/Documentation/git-merge-index.txt
index 4d266de..921b38f 100644
--- a/Documentation/git-merge-index.txt
+++ b/Documentation/git-merge-index.txt
@@ -8,7 +8,7 @@
 
 SYNOPSIS
 --------
-'git merge-index' [-o] [-q] <merge-program> (-a | [--] <file>\*)
+'git merge-index' [-o] [-q] <merge-program> (-a | [--] <file>*)
 
 DESCRIPTION
 -----------