fix example

Author: jeromevdl

examples/powertools-examples-core-utilities/sam/src/main/java/helloworld/App.java

@@ -67,7 +67,7 @@ public APIGatewayProxyResponseEvent handleRequest(final APIGatewayProxyRequestEv
 
         metricsLogger().putMetric("CustomMetric3", 1, Unit.COUNT, StorageResolution.HIGH);
 
-        LoggingUtils.appendKey("test", "willBeLogged");
+        MDC.put("test", "willBeLogged");
 
         APIGatewayProxyResponseEvent response = new APIGatewayProxyResponseEvent()
                 .withHeaders(headers);