[META] Improve ask user to sign challenge example

[s-n-park]

Is your feature request related to a problem? Please describe.
User gave feedback that the following is a bad example (potentially stale)
https://docs.near.org/build/web3-apps/backend/#2-ask-the-user-to-sign-the-challenge

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[thisisjoshford]

Looking into this and will test functionality.

[PiVortex]

@thisisjoshford A few other devs have reported issues with trying to use this recently.

Here is a related issue on an example references in the NEPs gagdiez/near-login#1

This message from a dev provides useful context https://t.me/neardev/58922