[website] Add note to Avalanche tutorial about only being able to have one Proof per Node

Summary:
I have receved a few questions from people asking if they can add more than one Proof to a Node.
So clarify this.

Test Plan: Ran it locally, read it through, looks fine

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12298