update link to Jupyter contributor guide
update link to Jupyter contributor guide

we only had the IPython placeholder from before the Jupyter contributor docs

jupyterhub/oauthenticator,jupyter/oauthenticator,NickolausDS/oauthenticator,minrk/oauthenticator,enolfc/oauthenticator,jupyter/oauthenticator,maltevogl/oauthenticator