Docs: environment variables link to be fixed

On this page:

https://tango-controls.readthedocs.io/projects/pytango/en/latest/tutorial.html#check-the-default-tango-host

we link to

https://tango-controls.readthedocs.io/en/latest/development/advanced/reference.html#environment-variables

while it should be (I think)

https://tango-controls.readthedocs.io/en/latest/Reference/reference.html#environment-variables