interop-test-descriptions.md: correct proto for custom_metadata's
FullDuplexCall
diff --git a/doc/interop-test-descriptions.md b/doc/interop-test-descriptions.md
index 96d1f2b..da3b976 100644
--- a/doc/interop-test-descriptions.md
+++ b/doc/interop-test-descriptions.md
@@ -716,7 +716,9 @@
 
     ```
     {
-      response_size: 314159
+      response_parameters:{
+        size: 314159
+      }
       payload:{
         body: 271828 bytes of zeros
       }