Example script in custom-checks is not considered to be a valid flux script

The task script in this page is not accepted in the task editor.

##### Relevant URLs
- https://docs.influxdata.com/influxdb/cloud/monitor-alert/custom-checks/
- _Provide other relevant URLs (project issues, community threads, etc.)_

Start with the custom-checks page at https://docs.influxdata.com/influxdb/cloud/monitor-alert/custom-checks/ and reproduce the task script in the task editor. Inspect the example and the editor's validation result, then update the documented example so it is accepted as a valid Flux script and verify the page instructions still match.

Written by the indexing model from the issue text.