Documentation/tutorial-2: Fix interesting typo in an example.

Marco Candrian noticed that one cat-file example refers to a
blob object that is never used in the example sequence.

The bug is interesting in that the output from the botched
sample command is consistent with the incorrect blob object
name ;-).

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

Documentation/tutorial-2.txt

@@ -343,8 +343,8 @@ And, as you can see with cat-file, this new entry refers to the
 current contents of the file:
 
 ------------------------------------------------
-$ git cat-file blob a6b11f7a
-goodbye, word
+$ git cat-file blob 8b9743b2
+goodbye, world
 ------------------------------------------------
 
 The "status" command is a useful way to get a quick summary of the