Fix molecule to run the correct Ubuntu 20.04 image (#82)

In our molecule test for ubuntu20, we were actually running an ubuntu18
image by mistake. This fixes it so that we're running an Ubuntu 20 image
in the ubuntu20 test.
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molecule/legacy/molecule.yml

@@ -26,7 +26,7 @@ platforms:
     privileged: true
     pre_build_image: true
   - name: ubuntu20
-    image: geerlingguy/docker-ubuntu1804-ansible
+    image: geerlingguy/docker-ubuntu2004-ansible
     command: ${MOLECULE_DOCKER_COMMAND:-""}
     volumes:
       - /sys/fs/cgroup:/sys/fs/cgroup:ro