Password reset screen asks for phone number in form that actually expects the username

[Rovack]

When a user forgets their password (or an admin resets it for them in Cognito), they get to a screen asking them to enter their phone number, so a reactivation code can be sent to it.
If they enter the phone number, however, they just get an error stating "Username/client id combination not found."

Apparently, this is due to a regression introduced in 70104f1.
Previously, the form really was expecting a phone number, but now it's expecting a username, yet the copy hasn't been updated.

So, we just need to update the label, placeholder, and paragraph at the top to let the user know they need to enter their username, not their phone number.