Fix module 'posixpath' has no attribute 'expand'

[cmd-ntrf]

I originally got the wrong function name for expanding user path.
https://docs.python.org/3/library/os.path.html#os.path.expanduser

[github-actions]

👈 Launch a binder notebook on this branch for commit 3842c49

I will automatically update this comment whenever this PR is modified

[consideRatio]

Thanks for catching and fixing this @cmd-ntrf!

[W 2023-09-22 22:04:40.387 ServerApp] jupyter_server_proxy | extension failed loading with message: AttributeError("module 'posixpath' has no attribute 'expand'")
    Traceback (most recent call last):
      File "/srv/conda/envs/notebook/lib/python3.10/site-packages/jupyter_server/extension/manager.py", line 356, in load_extension
        extension.load_all_points(self.serverapp)
      File "/srv/conda/envs/notebook/lib/python3.10/site-packages/jupyter_server/extension/manager.py", line 228, in load_all_points
        return [self.load_point(point_name, serverapp) for point_name in self.extension_points]
      File "/srv/conda/envs/notebook/lib/python3.10/site-packages/jupyter_server/extension/manager.py", line 228, in <listcomp>
        return [self.load_point(point_name, serverapp) for point_name in self.extension_points]
      File "/srv/conda/envs/notebook/lib/python3.10/site-packages/jupyter_server/extension/manager.py", line 219, in load_point
        return point.load(serverapp)
      File "/srv/conda/envs/notebook/lib/python3.10/site-packages/jupyter_server/extension/manager.py", line 147, in load
        return loader(serverapp)
      File "/srv/conda/envs/notebook/lib/python3.10/site-packages/jupyter_server_proxy/__init__.py", line 47, in _load_jupyter_server_extension
        server_pr