Rename postgresql_psycopg2 backend to postgresql in example

Django renamed this backup in upstream commit:
django/django@ec90047

Released as part of Django 1.9.

Author: jdufresne

example/settings.py

@@ -78,7 +78,7 @@
     # % createdb debug_toolbar -O debug_toolbar
     DATABASES = {
         "default": {
-            "ENGINE": "django.db.backends.postgresql_psycopg2",
+            "ENGINE": "django.db.backends.postgresql",
             "NAME": "debug_toolbar",
             "USER": "debug_toolbar",
         }