Update extension tutorial

[oruebel]

With the latest release of the ndx-template we should also check to make sure the extension tutorial is up-to-date. I believe the changes needed are minor, but, e.g., the directory has likely changed slightly due to the addition of GitHub actions etc.

https://nwb-overview.readthedocs.io/en/latest/extensions_tutorial/2_create_extension_spec_walkthrough.html#using-ndx-template