Fix code block in docs.

lib/spack/spack/multimethod.py

@@ -138,7 +138,7 @@ class when(object):
        methods like install() that depend on the package's spec.
        For example:
 
-       .. code-block::
+       .. code-block:: python
 
           class SomePackage(Package):
               ...
@@ -163,6 +163,8 @@ def install(self, prefix):
        if you only have part of the install that is platform specific, you
        could do this:
 
+       .. code-block:: python
+
           class SomePackage(Package):
               ...
               # virtual dependence on MPI.