Fix highlight languages in docs.

doc/ext/napoleon.rst

@@ -11,6 +11,8 @@
 Napoleon - *Marching toward legible docstrings*
 ===============================================
 
+.. highlight:: text
+
 Are you tired of writing docstrings that look like this::
 
     :param path: The path of the file to wrap
@@ -124,7 +126,9 @@ Napoleon supports two styles of docstrings: `Google`_ and `NumPy`_. The
 main difference between the two styles is that Google uses indention to
 separate sections, whereas NumPy uses underlines.
 
-Google style::
+Google style:
+
+.. code-block:: python3
 
     def func(arg1, arg2):
         """Summary line.
@@ -141,7 +145,9 @@ Google style::
         """
         return True
 
-NumPy style::
+NumPy style:
+
+.. code-block:: python3
 
     def func(arg1, arg2):
         """Summary line.