loki-mixin: Expose prometheus rules as prometheusRules instead of prometheus_rules

[sandhose]

The Prometheus rules defined in the mixin are under the prometheus_rules key. There's a design document that define how monitoring mixins should work, and it says prometheus rules should live under the prometheusRules key.

diff --git a/production/loki-mixin/recording_rules.libsonnet b/production/loki-mixin/recording_rules.libsonnet
index 7bf1f40..c4f121c 100644
--- a/production/loki-mixin/recording_rules.libsonnet
+++ b/production/loki-mixin/recording_rules.libsonnet
@@ -1,7 +1,7 @@
 local utils = import "mixin-utils/utils.libsonnet";
 
 {
-  prometheus_rules+:: {
+  prometheusRules+:: {
     groups+: [{
       name: 'loki_rules',
       rules: