Some documentation improvements
## Description
1. Fixed the dead URL to joblib
2. Fixed Stack Overflow brand name (with space)
### Types of change
Documentation
## Checklist
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tick off all the boxes. [] -> [x] -->
- [x] I have submitted the spaCy Contributor Agreement.
- [x] I ran the tests, and all new and existing tests passed.
- [x] My changes don't require a change to the documentation, or if they do, I've added all required information.
* Fix venv command examples
The documentation refers to `venv`, which is native to Python3.
However, the command examples are as if they were still `virtualenv`,
which is a package independent of `venv`:
- It doesn't need to be installed via `pip`. In fact `pip install venv` would
return an error.
- The correct way to invoke `venv` is `python3 -m venv`, not `venv`, which would
return command not found.
See https://docs.python.org/3/library/venv.html
I suspect the documentation simply replaced all occurrences of `virtualenv` with
`venv`. However they are different modules and are used differently.
* Update comment [ci skip]