Add example of yaml file with --extra-vars

[mjmayer]

SUMMARY

Add example in docs for using a yaml file with --extra-vars. From the docs it is not obvious that when using --extra-vars with a file the filename argument needs to be prefixed with @

This is already documented in the code. https://github.com/ansible/ansible/blob/devel/lib/ansible/cli/arguments/option_helpers.py#L363-L367

ISSUE TYPE

• Docs Pull Request

COMPONENT NAME

docs, playbook_variables.rst

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[samccann]

Thanks @mjmayer for the Ansible docs fix!