Fix typo re: pipelines for MRs & enable on .com

This corrects two issues for https://gitlab.com/gitlab-org/gitlab-ce/issues/15310:

• Typo with merge_requests keyword (was shown as merge-requests)
• Now that https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5792#note_126397969 is complete, update to show that this feature is available on .com