change the alert to trigger on cpu usage

simplexalerts/index.md

@@ -414,13 +414,18 @@ Now you need to configure it:
 - add an alerting condition (must be True to fire)
 - Use the preview button to check that the alert would indeed be firing upon creation
 
-#### A more useful alert
+#### Testing the alert
 
-Since the goal of this alert is to test the end to end notification pipeline it is built to be a minimal example.
+First install stress-ng:
 
-A more useful one would be the following for monitoring uptime of a server based on it's metrics endpoint reachability:
+    apt install stress-ng
 
-    absent(up{application="node",instance="myserver5496497897891561asdf.onion"})
+Then run
+
+    stress-ng --cpu 8
+
+Wait for at least five minutes so the rolling average correctly gets updated.
+If you have more than 8 cores it might be necessary to increase that number in order to produce the required CPU Usage.
 
 #### Keeping things Tidy