Fix displayname endpoint

mytrix.py

@@ -88,7 +88,7 @@ class Matrix:
 
     def set_displayname(self, nick):
         """Set the account's display name."""
-        endpoint = "/profile/%s/displayname"
+        endpoint = "/r0/profile/%s/displayname" % (self.matrix_id,)
         content = {"displayname": nick}
 
         self._put(endpoint, json_data=content)