Update sandbox.rst

Use the same option in the virtualenv command as the mkvirtualenv command uses.
apt-get is nowadays shortened to apt.

docs/source/internals/sandbox.rst

 In order to compile uWSGI, which is a dependency of the sandbox, you will
 first need to install the Python development headers with:::
 
-    $ sudo apt-get install python3-dev
+    $ sudo apt install python3-dev
 
 Install Oscar and its dependencies within a virtualenv:
 
 
 .. code-block:: bash
 
-    $ virtualenv oscar
+    $ virtualenv --python=python3 oscar
     $ source ./oscar/bin/activate
     (oscar) $