Improve MimirGossipMembersEndpointsOutOfSync runbook command

[colega]

What this PR does

Improves the command in the runbook to reduce the time to identify the issue.

I ran the previous command and the output is:

$ kubectl get endpoints gossip-ring
NAME          ENDPOINTS                                                                 AGE
gossip-ring   10.200.100.64:7946,10.200.100.65:7946,10.200.101.201:7946 + 979 more...   462d

While the new one just prints:

$ kubectl get endpoints gossip-ring -ojson | jq '.subsets[].addresses | length'
982

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.