Fixed wrong Pull Requests Activities path (#1326)

Author: roboqo

atlassian/bitbucket/__init__.py

@@ -1781,7 +1781,7 @@ def get_pull_requests_activities(
         :param start:
         :return:
         """
-        url = "{}/activities".format(self._url_pull_request(project_key, repository_slug, pull_request_id))
+        url = "{}/activity".format(self._url_pull_request(project_key, repository_slug, pull_request_id))
         params = {}
         if start:
             params["start"] = start