fix edit on github button

[Kasper Kondzielski]

Despite provided configuration edit on gh button redirects to generated sources

# configure edit on github: https://docs.readthedocs.io/en/latest/guides/vcs.html
html_context = {
    'display_github': True, # Integrate GitHub
    'github_user': 'softwaremill', # Username
    'github_repo': 'diffx', # Repo name
    'github_version': 'master', # Version
    'conf_py_path': '/docs-sources/', # Path in the checkout to the docs root
}

[Kasper Kondzielski]

Done