docs: cleanup the explanation paragraphs at the start of each exercise.

exercises/error_handling/errors3.rs

@@ -1,8 +1,11 @@
 // errors3.rs
+//
 // This is a program that is trying to use a completed version of the
 // `total_cost` function from the previous exercise. It's not working though!
 // Why not? What should we do to fix it?
-// Execute `rustlings hint errors3` or use the `hint` watch subcommand for a hint.
+//
+// Execute `rustlings hint errors3` or use the `hint` watch subcommand for a
+// hint.
 
 // I AM NOT DONE