Fix broken Jupyter contributing guide link
# Contributing to BinderHub
Welcome! As a [Jupyter](https://jupyter.org) project, we follow the
-[Jupyter contributor guide](https://jupyter.readthedocs.io/en/latest/contributor/content-contributor.html).
+[Jupyter contributor guide](https://jupyter.readthedocs.io/en/latest/contributing/content-contributor.html).
There are several different setups for developing BinderHub, depending on which
parts of it you want to change: the [documentation](#documentation-changes),
