Edited vague step in Triage Rotation page

The current first step for the pings to slack channel public_merge_requests_referencing_confidential_issues do not make sense to me. I'm editing it to be clearer and simpler. Please LMK if you disagree and / or would like the change to be something else. The "link to the comment" part seems quite unclear as to what it means. Simply adding a reply to the Slack thread should suffice.