git-update-index(1): fix use of quoting in section title

Signed-off-by: NJonas Fonseca <fonseca@diku.dk>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-update-index.txt

@@ -216,8 +216,8 @@ $ git ls-files -s
 ------------
 
 
-Using "assume unchanged" bit
-----------------------------
+Using ``assume unchanged'' bit
+------------------------------
 
 Many operations in git depend on your filesystem to have an
 efficient `lstat(2)` implementation, so that `st_mtime`