Update howto on PyPI to match CANFAR docs page

[mtbannister]

Please update the wording of https://pypi.org/project/vos/ so that it matches https://www.canfar.net/en/docs/storage/ . e.g. getCert is now depreciated in favour of cadc-get-cert , and thus step 3 of the PyPI tutorial leads to a lot of hunting for what to try next. Step 3 would be e.g. helpfully phrased as:
At the command line,

cadc-get-cert -u YOURUSERNAME
and enter your password.