docs: clarify how to activate virtualenv from mod_wsgi with py3

activate_this.py is not available with the venv module in py3. According to
https://modwsgi.readthedocs.io/en/develop/user-guides/virtual-environments.html
we just need to set python-home - that is the way to do proper virtualenv
activation, and there is no need for hacks in the wrapper script.