Support previous urls installation.

Update the __init__.py module to use the proper urls string path and app name.
This will maintain backwards compatability with the documentations previous
installation instructions.

debug_toolbar/__init__.py

@@ -1,5 +1,7 @@
 import django
 
+from debug_toolbar.urls import app_name
+
 __all__ = ["VERSION"]
 
 
@@ -9,7 +11,7 @@
 
 # Code that discovers files or modules in INSTALLED_APPS imports this module.
 
-urls = "debug_toolbar.toolbar", "djdt"
+urls = "debug_toolbar.urls", app_name
 
 if django.VERSION < (3, 2):
     default_app_config = "debug_toolbar.apps.DebugToolbarConfig"