Notes retirement endpoint should require trailing slash

This is because we already require trailing slashes on most of the rest
of the retirement endpoints, which are called via slumber which appends
the trailing slash by default.

lms/djangoapps/edxnotes/api_urls.py

@@ -6,5 +6,5 @@ from django.conf.urls import url
 from edxnotes import views
 
 urlpatterns = [
-    url(r"^retire_user$", views.RetireUserView.as_view(), name="edxnotes_retire_user"),
+    url(r"^retire_user/$", views.RetireUserView.as_view(), name="edxnotes_retire_user"),
 ]