Fix grammar in health_check.md

A access token -> An access token

doc/monitoring/health_check.md

@@ -9,7 +9,7 @@ and access the cache. This endpoint can be provided to uptime monitoring service
 
 ## Access Token
 
-A access token needs to be provided while accessing the health check endpoint. The current
+An access token needs to be provided while accessing the health check endpoint. The current
 accepted token can be found on the `admin/heath_check` page of your GitLab instance.
 
 ![access token](img/health_check_token.png)