updates to quickstart page

[kwmonroe]

couple updates for the quickstart page:

https://ubuntu.com/kubernetes/docs/quickstart

• step 1 asciinema shows --classic in the juju snap install command
• step 5 adds a model named k8s; i think this will trip up users who later run juju add-k8s and confuse what model does what; suggest we call this my-model following the my-controller pattern from step 4
• step 6 asciinema output shows cs:~containers charms

[evilnick]

I think we need a broader discussion about what we tell users to name things, because I also think there are issues with calling stuff 'my-model' (and 'my-controller')

The asciinema stuff is fine, I think i can just edit the actual files