git-show-ref doc: typeset regexp in fixed width font

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show-ref.txt

@@ -73,7 +73,7 @@ OPTIONS
 --exclude-existing[=<pattern>]::
 
 	Make 'git show-ref' act as a filter that reads refs from stdin of the
-	form "{caret}(?:<anything>\s)?<refname>(?:{backslash}{caret}{})?$"
+	form "`{caret}(?:<anything>\s)?<refname>(?:{backslash}{caret}{})?$`"
 	and performs the following actions on each:
 	(1) strip "{caret}{}" at the end of line if any;
 	(2) ignore if pattern is provided and does not head-match refname;