Fix protobuf README snippet validation

Author: jwils

elasticgraph-protobuf/README.md

@@ -64,6 +64,8 @@ index 2943335..26633c3 100644
 Then opt into proto generation from your schema definition:
 
 ```ruby
+# in config/schema/protobuf.rb
+
 ElasticGraph.define_schema do |schema|
   schema.proto_schema_artifacts package_name: "myapp.events.v1"
 end