Clarify password prompt

[cellularmitosis]

Hey fastlane team!

First, thanks so much for working on these tools!

I just tried using xcversion and was presented with a prompt. What username is this asking for? Can we provide some context via a prompt?

$ xcversion --verbose update
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The login information you enter will be stored in your Mac OS Keychain
You can also pass the password using the `FASTLANE_PASSWORD` environment variable
More information about it on GitHub: https://github.com/fastlane/fastlane/tree/master/credentials_manager
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Username: 

[KrauseFx]

Great question, this is your Apple Developer Portal username, as Xcode Betas are behind a paywall for the Developer Membership. Feel free to submit a PR making this easier for new users 👍