fix distribution example

Author: SylvainJuge

examples/distro/instrumentation/servlet-3/src/main/java/com/example/javaagent/instrumentation/DemoServlet3HelperClass.java

@@ -6,10 +6,13 @@
 package com.example.javaagent.instrumentation;
 
 import com.example.javaagent.bootstrap.AgentApi;
+import io.opentelemetry.instrumentation.api.util.VirtualField;
+import java.util.concurrent.atomic.AtomicInteger;
+import javax.servlet.ServletResponse;
 
 public final class DemoServlet3HelperClass {
 
-  public final VirtualField<ServletResponse, AtomicInteger> VIRTUAL_FIELD =
+  public static final VirtualField<ServletResponse, AtomicInteger> VIRTUAL_FIELD =
       VirtualField.find(ServletResponse.class, AtomicInteger.class);
 
   public static void doSomething(int number) {

examples/distro/instrumentation/servlet-3/src/main/java/com/example/javaagent/instrumentation/DemoServlet3Instrumentation.java

@@ -47,7 +47,7 @@ public static void onEnter(@Advice.Argument(value = 1) ServletResponse response)
       AtomicInteger counter = DemoServlet3HelperClass.VIRTUAL_FIELD.get(response);
       if (counter == null) {
         counter = new AtomicInteger();
-        virtualField.set(response, counter);
+        DemoServlet3HelperClass.VIRTUAL_FIELD.set(response, counter);
       }
       DemoServlet3HelperClass.doSomething(counter.incrementAndGet());