README.md: adapt for new Ubuntu box

Author: miri64

README.md

@@ -26,7 +26,7 @@ For links go to [https://github.com/RIOT-OS/Tutorials](https://github.com/RIOT-O
     cd Tutorials
     git submodule update --init --recursive
     ```
-* In case a virtual machine is disseminated locally, adapt the path for the vagrant box `vagrant box add RIOT/ubuntu1604 <path to box>`
+* In case a virtual machine is disseminated locally, adapt the path for the vagrant box `vagrant box add RIOT/ubuntu1804 <path to box>`
 * Run `vagrant up` and `vagrant ssh` afterwards. See the [Vagrant RIOT Setup](https://github.com/RIOT-OS/RIOT/blob/master/dist/tools/vagrant/README.md) for a more general explanation.
 
 **Regular Setup without using a VM (recommended for RIOT developement)**