Documentation: retitle the git-core tutorial

Give the git-core tutorial a name that better reflects its intended
audience.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/core-tutorial.txt

@@ -1,5 +1,5 @@
-A short git tutorial
-====================
+A git core tutorial for developers
+==================================
 
 Introduction
 ------------