Update health_check.md - Fix spelling of admin/health_check path. [ci skip]
What does this MR do?
Fix the spelling of admin/heath_check to admin/health_check in health_check documentation
Are there points in the code the reviewer needs to double check?
No
Why was this MR needed?
Fix a spelling error that may mislead a user on the proper URL/Path to check the GitLab health.