Oprava příkazu pro instalaci virtualenv (#40)

Author: PetrEconomist

Diff for: lessons/beginners/install/linux.md

@@ -103,7 +103,7 @@ alternativu, Virtualenv:
 * **Ubuntu**:
   {% filter markdown(inline=True) %}
   ```console
-  $ sudo apt-get install python-virtualenv
+  $ sudo apt-get install python3-virtualenv
   ```
   {% endfilter %}