[TRIVIAL] Make the release notes mardown link project absolute
[TRIVIAL] Make the release notes mardown link project absolute

Summary: Using a project absolute (relative to project root)  link will make sure the link remains valid after the release notes is archived.

Test Plan: Proof reading.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11116

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