Documentation/etcd-mixin: fix typo in README.md

Promethues -> Prometheus

Documentation/etcd-mixin/README.md

@@ -1,4 +1,4 @@
-# Promethues Monitoring Mixin for etcd
+# Prometheus Monitoring Mixin for etcd
 
 > NOTE: This project is *alpha* stage. Flags, configuration, behaviour and design may change significantly in following releases.