Updates Firewalls create example with tags

[dbrian57]

Addresses user feedback about how to use tags when creating a firewall.

[loosla]

Looks great! Thanks a lot for the update 🚀
Could you please also update L71 🙏 with

		tagNameRulesTxt := "A comma-separated list of existing tags, for example: frontend,backend,env:prod. Droplets with these tags will be placed behind the cloud firewall"

It may be useful to include an example with ":" to the --tag-names doc.
Many thanks.

Done in af3a106