chore: In the 01_getting_started tutorial, clarify how to use gnokey add

[jefft0]

This is a small clarification in the 01_getting_started tutorial . The instructions say to execute the gnokey add command and enter the mnemonic. But the first prompt is "Enter a passphrase to encrypt your key to disk". For my first try (and other people that I've seen), they try to paste the mnemonic here which doesn't work.

This PR updates to make it more clear, similar to the other example for gnokey add at the bottom of the same tutorial.

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[thehowl]

Thanks!