Update streaming_rpc.md

docs/en/streaming_rpc.md

@@ -42,9 +42,9 @@ struct StreamOptions
     // default: -1
     long idle_timeout_ms;
      
-    // How many messages at most passed to handler->on_received_messages
-    // default: 1
-    size_t max_messages_size;
+    // Maximum messages in batch passed to handler->on_received_messages
+    // default: 128
+    size_t messages_in_batch;
  
     // Handle input message, if handler is NULL, the remote side is not allowd to
     // write any message, who will get EBADF on writting