Fix tutorial-2.html

Honza Pazdziora noticed that one example did not match reality.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/tutorial-2.txt

@@ -244,6 +244,7 @@ $ git ls-files --stage
 $ git cat-file -t 513feba2
 blob
 $ git cat-file blob 513feba2
+hello world!
 hello world, again
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