fix build error

r2objects.go

@@ -10,7 +10,9 @@ import (
 type R2Objects struct {
 	Objects   []*R2Object
 	Truncated bool
-	Cursor            *string
+	// Cursor indicates next cursor of R2Objects.
+	// * This becomes empty string if cursor doesn't exist.
+	Cursor            string
 	DelimitedPrefixes []string
 }