check-ref-format documentation: fix enumeration mark-up

The last item in the enumerated refname rule was mistakenly made into
a sub-item of the 7th one.  It should be the 8th one in the list on its
own.
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