Fix mismatched virtual environment names in tools page

[eliheuer]

Commit 0ace6a0 changed source env/bin/activate to source myenv/bin/activate but left the old env in some other parts of the tools section. I changed everything to myenv to make it less confusing.

I also replaced "venv" with "virtual environment" in the text a few times for clarity.

[RosaWagner]

Thanks @eliheuer !