Re-fix check-ref-format documentation mark-up

It is not double-backslash we forbid; backslashes are forbidden since
a4c2e699 (Disallow '\' in ref names, 2009-05-08)
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-check-ref-format.txt

@@ -43,7 +43,7 @@ imposes the following rules on how references are named:
 
 . They cannot contain a sequence `@{`.
 
-. They cannot contain a `\\`.
+. They cannot contain a `\`.
 
 These rules make it easy for shell script based tools to parse
 reference names, pathname expansion by the shell when a reference name is used