Update link for creating a release

[GeorgianaElena]

I spotted this while checking the release procedure.

[rgaiacs]

Thanks @GeorgianaElena for spot this.

We use sphinxext-rediraffe to avoid that links break. Add a entry to

repo2docker/docs/source/conf.py

Line 140 in e792c9b

# "old-file": "new-folder/new-file-name",

to avoid the 404 error for people that visit https://repo2docker.readthedocs.io/en/latest/contributing/tasks/#creating-a-release from achieve? I believe the line to be added is

"contributing/tasks.md": "contribute/tasks.md"