chore (samples) Added a place for updateMask insertion from tutorials (#498)

* Added comment for update mask.

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retail/interactive-tutorials/src/main/java/product/UpdateProduct.java

@@ -86,6 +86,8 @@ public static void updateProduct(Product originalProduct, String defaultBranchNa
               .build();
       System.out.printf("Update product request: %s%n", updateProductRequest);
 
+      // PASTE UPDATE MASK HERE: requires import com.google.protobuf.FieldMask
+
       Product updatedProduct = serviceClient.updateProduct(updateProductRequest);
       System.out.printf("Updated product: %s%n", updatedProduct);
     }