fix linter

pkg/commands/copy_test.go

@@ -413,7 +413,7 @@ func TestGetUserGroup(t *testing.T) {
 		{
 			description: "empty chown string",
 			mock: func(c string, t bool) (uint32, uint32, error) {
-				return 0, 0, fmt.Errorf("should not be called.")
+				return 0, 0, fmt.Errorf("should not be called")
 			},
 			expectedU: -1,
 			expectedG: -1,