Update docstring

Change-Id: I46181b3d575c3fa240f178f60e53a0e33bd11357

Author: devchas

google-ads-examples/src/main/java/com/google/ads/googleads/examples/campaignmanagement/ValidateAd.java

@@ -40,8 +40,8 @@
 import java.util.stream.Collectors;
 
 /**
- * Shows how to use the validateOnly header to validate a responsive search ad. No ads will be
- * created, but exceptions will still be thrown.
+ * Shows how to use the validateOnly request parameter to validate a responsive search ad. No ads
+ * will be created, but exceptions will still be thrown.
  */
 public class ValidateAd {