Update MR rate desc and link for gitlab.com availability slo trends metric
Link for https://about.gitlab.com/handbook/engineering/development/performance-indicators/#gitlabcom-availability-apdex-slo-trends is broken.
Update https://dashboards.gitlab.com/d/general-slas/general-slas?viewPanel=6&from=now-7d&orgId=1&to=now to https://dashboards.gitlab.net/d/general-slas/general-slas?from=now-7d&orgId=1&to=now
Edited by Lily Mai