fix: re-add missing done comments

exercises/strings/strings3.rs

@@ -1,6 +1,8 @@
 // strings3.rs
 // Execute `rustlings hint strings3` or use the `hint` watch subcommand for a hint.
 
+// I AM NOT DONE
+
 fn trim_me(input: &str) -> String {
     // TODO: Remove whitespace from the end of a string!
     ???