[COOK-2753] Update README to reflect changes in Django cookbook

The Django settings template was changed to use the keyword "adapter" instead of "engine", but the documentation still used the old keyword "engine".

README.md

@@ -117,7 +117,7 @@ application 'packaginator' do
     collectstatic     'build_static --noinput'
     database do
       database 'packaginator'
-      engine   'postgresql_psycopg2'
+      adapter  'postgresql_psycopg2'
       username 'packaginator'
       password 'awesome_password'
     end