Instructions for installing molecule in book missing the docker plugin installation

I saw you fixed the molecule [README](https://github.com/geerlingguy/ansible-for-devops/blob/master/molecule/README.md). I saw you fixed the [CI job](https://github.com/geerlingguy/ansible-for-devops/issues/545). I even see the automated CI job recipe later in the book installing it.

But I still see in the latest edition that the initial instructions for installing molecule is missing the plugin.
You have:
> Molecule is easy to install:
> ```bash
> pip3 install molecule
> ```

You proceed to create the `molecule.yml` file with the docker driver and call `molecule test`. In between, there are no steps to install the `molecule-plugins[docker]` package.

Got me puzzled for the weird message that molecule gives when it is not installed.

Locate the book section with the initial “Molecule is easy to install” instructions, using molecule/README.md and the later automated CI recipe as context. Add the molecule-plugins[docker] installation step before creating molecule.yml, then verify the documented molecule test flow is complete.

Written by the indexing model from the issue text.