Fix: change error code in function isAccessDenied to AccessDenied

Signed-off-by: Xun Jiang <blackpiglet@gmail.com>

changelog/unreleased/issue-4085

@@ -0,0 +1 @@
+Fix error code in function isAccessDenied.

internal/backend/s3/s3.go

@@ -164,7 +164,7 @@ func isAccessDenied(err error) bool {
 	debug.Log("isAccessDenied(%T, %#v)", err, err)
 
 	var e minio.ErrorResponse
-	return errors.As(err, &e) && e.Code == "Access Denied"
+	return errors.As(err, &e) && e.Code == "AccessDenied"
 }
 
 // IsNotExist returns true if the error is caused by a not existing file.