correct link to kinematic demo for the stable docs

learning/features/physics/kinematic_character_2d.rst

@@ -246,4 +246,4 @@ certain threshold), the character can be determined to be there.
 
 A more complete demo can be found in the demo zip distributed with the
 engine, or in the
-https://github.com/godotengine/godot-demo-projects/tree/master/2d/kinematic_char.
+https://github.com/godotengine/godot-demo-projects/tree/2.1/2d/kinematic_char.