[release-process] Update Ubuntu PPA instruction
[release-process] Update Ubuntu PPA instruction

Summary: Should have been updated with D5552

Test Plan: Read it. Compare to `contrib/releases/debian-packages.sh --help`

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5730

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