Minor documentation example fix; bold and italic should be spelled out explicitly. (#3779)

docs/advanced_topics/customisation/page_editing_interface.rst

@@ -72,7 +72,7 @@ This can be achieved by passing a ``features`` keyword argument to ``RichTextFie
 
 .. code-block:: python
 
-    body = RichTextField(features=['h2', 'h3', 'b', 'i', 'link'])
+    body = RichTextField(features=['h2', 'h3', 'bold', 'italic', 'link'])
 
 The recognised feature identifiers are as follows (note that add-on modules may add to this list):