The 'hub' command fails now with the following error:

  Error creating pull request: Unauthorized (HTTP 401)
  Requires authentication
  github.com username: github.com password for  (never stored):

and it does not create a pull request with an automatic update
of documentation.

GitHub token has to be stored in the 'GITHUB_TOKEN' environment variable,
so that authentication of the 'hub' command worked correctly.