Changed wording

src/main.rs

@@ -372,7 +372,7 @@ fn main() -> Result<()> {
 
     let text = String::from_utf8(buf).unwrap_or_else(|_| {
         eprintln!(
-            "{}: {}: File contents are invalid UTF-8.", argv[0], argv[1]
+            "{}: {}: File contents are not valid UTF-8.", argv[0], argv[1]
         );
         exit(EX_DATAERR);
     });