fix a typo in alerting doc

[twelch24]

Please give a short description what your pull request is for

DO NOT DELETE THE UNDERLYING TEXT

Please note

Please read this information carefully. You can run ./lnms dev:check to check your code before submitting.

• Have you followed our code guidelines?
• If my Pull Request does some changes/fixes/enhancements in the WebUI, I have inserted a screenshot of it.
• If my Pull Request makes discovery/polling/yaml changes, I have added/updated test data.

Testers

If you would like to test this pull request then please run: ./scripts/github-apply <pr_id>, i.e ./scripts/github-apply 5926
After you are done testing, you can remove the changes with ./scripts/github-remove. If there are schema changes, you can ask on discord how to revert.

[librenms-bot]

This pull request has been mentioned on LibreNMS Community. There might be relevant details there:

https://community.librenms.org/t/22-5-0-changelog/18803/1