nixos/installer: update getty help message

I think this is clearer.

Changed files
+3 -3
nixos
modules
+3 -3
nixos/modules/profiles/installation-device.nix
···
services.getty.helpLine = ''
The "nixos" and "root" accounts have empty passwords.
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An ssh daemon is running. You then must set a password
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for either "root" or "nixos" with `passwd` or add an ssh key
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to /home/nixos/.ssh/authorized_keys be able to login.
+
To log in over ssh you must set a password for either "nixos" or "root"
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with `passwd` (prefix with `sudo` for "root"), or add your public key to
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/home/nixos/.ssh/authorized_keys or /root/.ssh/authorized_keys.
If you need a wireless connection, type
`sudo systemctl start wpa_supplicant` and configure a