alter type mismatch (#29)

diff --git a/protocol/dubbo/utils/buffer.go b/protocol/dubbo/utils/buffer.go
index d499a4d..f8790c1 100644
--- a/protocol/dubbo/utils/buffer.go
+++ b/protocol/dubbo/utils/buffer.go
@@ -113,7 +113,7 @@
 	if index <= b.capacity {
 		b.wrInd = index
 	} else {
-		return &BaseError{fmt.Sprintf("Index(%d) over the capacity(%s)", index, b.capacity)}
+		return &BaseError{fmt.Sprintf("Index(%d) over the capacity(%d)", index, b.capacity)}
 	}
 	return nil
 }