fix: re-add missing done comments

exercises/clippy/clippy3.rs

@@ -1,6 +1,8 @@
 // clippy3.rs
 // Here's a couple more easy Clippy fixes, so you can see its utility.
 
+// I AM NOT DONE
+
 #[allow(unused_variables, unused_assignments)]
 fn main() {
     let my_option: Option<()> = None;

exercises/quiz2.rs

@@ -18,6 +18,8 @@
 // - The output element is going to be a Vector of strings.
 // Execute `rustlings hint quiz2` or use the `hint` watch subcommand for a hint.
 
+// I AM NOT DONE
+
 pub enum Command {
     Uppercase,
     Trim,

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!
     ???