Make release-process.md IBD instruction more precise

Summary: Putting the actual command needed is more helpful.

Test Plan: Did IBD with these arguments, and checked debug.log to see that it skipped checkpoints and assumevalid.

Reviewers: deadalnix, markblundeberg, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3062