doc: fix unmatched code fences

This mismatch upsets the renderer on git-scm.com.

Signed-off-by: Jean-Noel Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitcore-tutorial.txt

@@ -259,7 +259,7 @@ index 557db03..263414f 100644
 @@ -1 +1,2 @@
  Hello World
 +It's a new day for git
-----
+------------
 
 i.e. the diff of the change we caused by adding another line to `hello`.