Fixed #25681 -- Added 'default' to DATABASES example.

docs/topics/db/multi-db.txt

@@ -249,6 +249,7 @@ with two read replicas. Here are the settings specifying these
 databases::
 
     DATABASES = {
+        'default': {},
         'auth_db': {
             'NAME': 'auth_db',
             'ENGINE': 'django.db.backends.mysql',