Wrong help text for login

[Tom Wardill]

On trying to create an override for a snap, without having logged in:

> vagrant@ubuntu-xenial:~/snapstore-snap$ snapstore-client override hello-atomatt-1 edge 80
No store configuration found. Have you run "snapstore login"?

> vagrant@ubuntu-xenial:~/snapstore-snap$ snapstore login
usage: snapstore [-h] [-d] COMMAND ...
snapstore: error: argument COMMAND: invalid choice: 'login' (choose from 'help', 'migrate', 'config', 'import-certificate', 'register', 'status', 'registration', 'list-overrides', 'override', 'delete-override', 'add-admin', 'remove-admin', 'generate-keys', 'create-database', 'check-connections')

The help text should contain 'snapstore-client login'