docs: move the import statement of debug_toolbar_urls to inside of the if statement for avoiding potential errors

Author: Lidoca

docs/installation.rst

@@ -183,9 +183,10 @@ You should also modify your URLconf file:
 .. code-block:: python
 
     from django.conf import settings
-    from debug_toolbar.toolbar import debug_toolbar_urls
 
     if not settings.TESTING:
+        from debug_toolbar.toolbar import debug_toolbar_urls
+
         urlpatterns = [
             *urlpatterns,
         ] + debug_toolbar_urls()