Make modifications clearer per Apache license

src/uu/sort/src/ext_sorter.rs

@@ -1,5 +1,5 @@
 // Copyright 2018 Andre-Philippe Paquet
-// Copyright 2021 Robert Swinford
+// Modifications copyright 2021 Robert Swinford
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -13,6 +13,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// This file has been modified for use in the uutils project.
+
 use rayon::prelude::*;
 use std::{
     cmp::Ordering,