Fix typo in tutorial-2.txt

This should be obvious enough.

I didn't actually _test_ the tutorial, but if the old command worked,
something is really wrong!

Signed-off-by: Linus "Duh!" Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/tutorial-2.txt

diff --git a/Documentation/tutorial-2.txt b/Documentation/tutorial-2.txt
index 82c6922..894ca5e 100644
--- a/Documentation/tutorial-2.txt
+++ b/Documentation/tutorial-2.txt
@@ -136,7 +136,7 @@
 ------------------------------------------------
 $ git ls-tree d0492b36
 100644 blob a0423896973644771497bdc03eb99d5281615b51    file.txt
-$ git cat-file commit a0423896
+$ git cat-file blob a0423896
 hello world!
 ------------------------------------------------