Fix producer's JSON output

sensors/producer/index.js

@@ -21,7 +21,7 @@ async function main({id}) {
     topic: 'mra5q18k-sensors',
     messages: [{
       key: null,
-      value: `{producer: ${id}, timestamp: ${date}, value: ${value}}`}]
+      value: `{"producer": ${id}, "timestamp": ${date}, "value": ${value}}`}]
   })
   await producer.disconnect()