Update EcsEncoder.java

Fixed per the feedback from the public PR.

Signed-off-by: Difrango, Ronald <Ronald.Difrango@fmr.com>

logback-ecs-encoder/src/main/java/co/elastic/logging/logback/EcsEncoder.java

@@ -142,7 +142,7 @@ private void serializeMessage(ILoggingEvent event, StringBuilder builder) {
         if (messageLayout == null) {
             EcsJsonSerializer.serializeFormattedMessage(builder, event.getFormattedMessage());
         } else {
-            EcsJsonSerializer.serializeFormattedMessage(builder, this.messageLayout.doLayout(event));
+            EcsJsonSerializer.serializeFormattedMessage(builder, messageLayout.doLayout(event));
         }
     }