Fixes missing pre block in configuration docs

docs/configuration.rst

@@ -258,7 +258,7 @@ prefix:
 
     CELERY_RESULT_BACKEND = 'db+scheme://user:password@host:port/dbname'
 
-Examples:
+Examples::
 
     # sqlite (filename)
     CELERY_RESULT_BACKEND = 'db+sqlite:///results.sqlite'