tutorial: use "project history" instead of "changelog" in header

The word "changelog" seems a little too much like jargon to me, and beginners
must understand section headers so they know where to look for help.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>

Documentation/tutorial.txt

@@ -132,8 +132,8 @@ and newly modified files, and in both cases it takes a snapshot of the
 given files and stages that content in the index, ready for inclusion in
 the next commit.
 
-Viewing the changelog
+Viewing project history
----------------------
+-----------------------
 
 At any point you can view the history of your changes using