fix(traits): update hint comments

exercises/traits/traits3.rs

@@ -5,6 +5,7 @@
 // information without writing the same function twice.
 //
 // Consider what you can add to the Licensed trait.
+// Execute `rustlings hint traits3` or use the `hint` watch subcommand for a hint.
 
 // I AM NOT DONE
 

exercises/traits/traits4.rs

@@ -2,6 +2,7 @@
 //
 // Your task is to replace the '??' sections so the code compiles.
 // Don't change any line other than 21.
+// Execute `rustlings hint traits4` or use the `hint` watch subcommand for a hint.
 
 // I AM NOT DONE
 

exercises/traits/traits5.rs

@@ -2,6 +2,7 @@
 //
 // Your task is to replace the '??' sections so the code compiles.
 // Don't change any line other than 27.
+// Execute `rustlings hint traits5` or use the `hint` watch subcommand for a hint.
 
 // I AM NOT DONE