Improve the placement of TODO comments

exercises/00_intro/intro1.rs

@@ -1,5 +1,5 @@
-// We sometimes encourage you to keep trying things on a given exercise, even
+// TODO: We sometimes encourage you to keep trying things on a given exercise,
-// after you already figured it out. If you got everything working and feel
+// even after you already figured it out. If you got everything working and feel
 // ready for the next exercise, enter `n` in the terminal.
 //
 // The exercise file will be reloaded when you change one of the lines below!

exercises/00_intro/intro2.rs

@@ -1,5 +1,4 @@
-// TODO: Fix the code to print "Hello world!".
-
 fn main() {
+    // TODO: Fix the code to print "Hello world!".
     printline!("Hello world!");
 }

exercises/01_variables/variables4.rs

@@ -1,5 +1,4 @@
 // TODO: Fix the compiler error.
-
 fn main() {
     let x = 3;
     println!("Number {x}");