Update sandbox.rst
Update sandbox.rst

Use the same option in the virtualenv command as the mkvirtualenv command uses.
apt-get is nowadays shortened to apt.
solarissmoke/django-oscar,django-oscar/django-oscar,solarissmoke/django-oscar,solarissmoke/django-oscar,django-oscar/django-oscar,solarissmoke/django-oscar,django-oscar/django-oscar,django-oscar/django-oscar