Fixed some embarrassing grammar

flask/blueprints.py

@@ -167,7 +167,7 @@ class Blueprint(_PackageBoundObject):
         the :func:`url_for` function is prefixed with the name of the blueprint.
         """
         if endpoint:
-            assert '.' not in endpoint, "Blueprint endpoint's should not contain dot's"
+            assert '.' not in endpoint, "Blueprint endpoints should not contain dots"
         self.record(lambda s:
             s.add_url_rule(rule, endpoint, view_func, **options))