Fix typo

server.go

@@ -15,7 +15,7 @@ import (
 
 var (
 	addr              = flag.String("addr", getenv("ADDR", ":3000"), "TCP `addr` to listen on")
-	karmaBackend      = flag.String("karma-url", getenv("ALERTS_URL", ""), "`URL` for the Karma backend")
+	karmaBackend      = flag.String("karma-url", getenv("KARMA_URL", ""), "`URL` for the Karma backend")
 	prometheusBackend = flag.String("prometheus-url", getenv("PROMETHEUS_URL", ""), "`URL` for the Prometheus backend")
 
 	rootDir = "/var/www"