release_process: make more links copy-paste-able
Originally created by @CyrilBrulebois on #16596 (Redmine)
Currently there are several links that can’t be pasted to a browser directy:
* APT_repository/time-based snapshots#freeze-exceptions-post-release
(×1)
* APT_repository/time-based snapshots#thaw
(×2)
Any objections to replacing the space with an underscore?