fix(traits4.rs): wrong line number

fixes #1067

exercises/traits/traits4.rs

@@ -1,7 +1,7 @@
 // traits4.rs
 //
 // Your task is to replace the '??' sections so the code compiles.
-// Don't change any line other than 21.
+// Don't change any line other than the marked one.
 // Execute `rustlings hint traits4` or use the `hint` watch subcommand for a hint.
 
 // I AM NOT DONE
@@ -19,6 +19,7 @@ struct OtherSoftware {}
 impl Licensed for SomeSoftware {}
 impl Licensed for OtherSoftware {}
 
+// YOU MAY ONLY CHANGE THE NEXT LINE
 fn compare_license_types(software: ??, software_two: ??) -> bool {
     software.licensing_info() == software_two.licensing_info()
 }