Update contributing guide to use git-pull

[fperez]

The contribution guide still lists git-sync, which if I understand correctly, is a legacy alias for git-pull. This updates the docs.

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[consideRatio]

From this code, I conclude you are correct that it is git-pull that is the non-deprecated URL!

def load_jupyter_server_extension(nbapp):
    web_app = nbapp.web_app
    base_url = url_path_join(web_app.settings['base_url'], 'git-pull')
    handlers = [
        (url_path_join(base_url, 'api'), SyncHandler),
        (base_url, UIHandler),
        (url_path_join(web_app.settings['base_url'], 'git-sync'), LegacyGitSyncRedirectHandler),
        (url_path_join(web_app.settings['base_url'], 'interact'), LegacyInteractRedirectHandler),
        (
            url_path_join(base_url, 'static', '(.*)'),
            StaticFileHandler,
            {'path': os.path.join(os.path.dirname(__file__), 'static')}
        )
    ]
    web_app.settings['nbapp'] = nbapp
    web_app.add_handlers('.*', handlers)

Thanks @fperez ❤️ 🎉!

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Congrats on your first merged pull request in this project! 🎉

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[fperez]

Thanks for the fast turnaround @consideRatio!