Improve highlighting of some code blocks. (GH-6401)

diff --git a/Doc/howto/urllib2.rst b/Doc/howto/urllib2.rst
index c1fd5cf..204a05a 100644
--- a/Doc/howto/urllib2.rst
+++ b/Doc/howto/urllib2.rst
@@ -457,7 +457,9 @@
 and a 'realm'. The header looks like: ``WWW-Authenticate: SCHEME
 realm="REALM"``.
 
-e.g. ::
+e.g.
+
+.. code-block:: none
 
     WWW-Authenticate: Basic realm="cPanel Users"