#11 fix package name in install instructions

Quote from the install doc:

On a Debian based distribution, do::

  $ sudo apt-get install virtualenv

On trisquel 7, there is no virtualenv package. The package name is python-virtualenv. On newer debian installs, the virtualenv package is dependent on python3-virtualenv, the python 3 version of virtualenv, not sure you want that.

Thanks for reporting this @iank!

I've pushed a fix.