cookbook: Remove bold on emphasis inside programlisting

If an <emphasis> element is placed inside a <programlisting> in a cookbook
recipe, the result is bold italic text in the HTML output. This isn't
particularly readable.

Fix is to style emphasis elements inside programlistings
so the font weight is not bold but is still italicised.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2475

doc/common/style.css

@@ -530,6 +530,9 @@ div.navfooter hr {
   font-weight: bold;
 }
 
+pre.programlisting .emphasis {
+  font-weight: normal !important;
+}
 
   /************* /
  / decorations  /