Update alerting_rules.md (#7252)
Update alerting_rules.md (#7252)

Update `ALERTS` example be more meta-syntactic.

Signed-off-by: Justin Walz <19f9d44fc3a88a6004fa86a1695cbc2b868fac69@gmail.com>
GoogleCloudPlatform/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,alileza/prometheus,alileza/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus