docs(firebase_ui_auth): fix phone provider's widget example

[yamhoresh]

Added handling for the auth state of SMSCodeRequested in the custom AuthFlowBuilder example, matching the implementation of the PhoneInputView.

Description

This PR includes only a small documentation edit for the phone auth provider in the firebase_ui_auth package.
The example for building a custom widget with AuthFlowBuilder, doesn't handle the auth state for SMSCodeRequested. When copy-pasting this example, it temporarily returns an unknown state text widget during login (see image below). I've fixed the example, modifying it based on the condition in the PhoneInputView widget.

Related Issues

None.

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[Lyokone]

Thanks for the PR :)