fix indentation on index page

This is how it currently looks:

content/index.rst

@@ -50,8 +50,8 @@ The complete code that constructs the graphene flake shown on the right side is
 .. code:: python
 
           def disk(pos):
-          x, y = pos
-          return x**2 + y**2 < 8**2
+              x, y = pos
+              return x**2 + y**2 < 8**2
 
           lat = kwant.lattice.honeycomb()
           sys = kwant.Builder()