fix: fix in metal_device inventory docs and tests

[antoninrykalsky]

Fixes documentation of usage of dynamic inventory. Based on issue #121

[displague]

I think this is the second test, based on the test failure not including a name:

TASK [assert] ******************************************************************
fatal: [localhost]: FAILED! => {
    "assertion": "'ansible-test-inventory' in hostvars",
    "changed": false,
    "evaluated_to": false,
    "msg": "Assertion failed"
}

[github-actions]

This PR is included in version 0.4.1 🎉