jsonnet: fix copy-paste errors, format, and lint

Signed-off-by: paulfantom <pawel@krupa.net.pl>

jsonnet/kube-prometheus/mixins/monitoring-mixins.libsonnet

@@ -1,8 +1,7 @@
 local defaults = {
-  local defaults = self,
   namespace: error 'must provide namespace',
-  prometheusName: error 'must provide namespace',
-  alertmanagerName: error 'must provide namespace',
+  prometheusName: error 'must provide Prometheus resource name',
+  alertmanagerName: error 'must provide Alertmanager resource name',
 };
 
 function(params) {