Move content_panels inside BookPage definition.

docs/topics/snippets.rst

@@ -110,8 +110,7 @@ In the above example, the list of adverts is a fixed list, displayed as part of
           related_name='+'
       )
   
-  
-  BookPage.content_panels = [
+      content_panels = Page.content_panels + [
           SnippetChooserPanel('advert'),
           # ...
       ]
@@ -152,8 +151,7 @@ To attach multiple adverts to a page, the ``SnippetChooserPanel`` can be placed
   class BookPage(Page):
       ...
   
-  
-  BookPage.content_panels = [
+      content_panels = Page.content_panels + [
           InlinePanel('advert_placements', label="Adverts"),
           # ...
       ]