[avalanche] Fix comment related to proof requests
[avalanche] Fix comment related to proof requests

Summary: Apparently a copy-paste from the related transactions request code immediately below it.

Test Plan: Read the comment and verify it makes sense.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11841

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