Fix config filename in config via traitlets doc

[cmd-ntrf]

The file has to be named jupyter_server_config.py when using JupyterLab and version of Notebook >= 6.5.

This is because jupyter-server is replacing the notebook backend: https://jupyter-server.readthedocs.io/en/latest/operators/migrate-from-nbserver.html

It might be better to actually explain how the file should be named based on the version used, but I wanted to provide a starting point for the conversation.

[hoogenm]

Useful info (correct filename for notebook >= 6.5).

[consideRatio]

Thanks @cmd-ntrf!! :)