Improve CredentialsSettingsVars documentation

[Niogui]

I think it would be a good idea to update the README.md of ntc-nautobot-plugin-nornir to replace the sample config:

PLUGINS_CONFIG = {
    "nautobot_plugin_nornir": {
        # ...
        "dispatcher_mapping": None,
        "username": "ntc",
        "password": "password123",
        "secret": "password123",
    }
}

with

PLUGINS_CONFIG = {
    "nautobot_plugin_nornir": {
        "nornir_settings": {
            "credentials": "nautobot_plugin_nornir.plugins.credentials.settings_vars.CredentialsSettingsVars
            # ...
        },
        # ...
        "dispatcher_mapping": None,
        "username": "ntc",
        "password": "password123",
        "secret": "password123",
    }
}

It would be much more explicit and avoid some mistake

[Niogui]

And you can also fix a typo in the previous bloc of code:

        "use_config_context": {"secrets", False, "connection_options": True},

should be:

        "use_config_context": {"secrets": False, "connection_options": True},

comma after secrets to be replaced by colons

[JulioPDX]

@jeffkala This issue reflects the changes that could be made to simplify or reduce errors. Thank you