doc: enchane string3 exercise hint

[yapjiahong]

enhance string3 exercise' hint

Add a replace_method hint; I think it would be more helpful for beginners.

• before

  

• after

  

• string3 exercise

  
  fn trim_me(input: &str) -> &str {
  // TODO: Remove whitespace from both ends of a string.
  }

fn compose_me(input: &str) -> String { // TODO: Add " world!" to the string! There are multiple ways to do this. }

fn replace_me(input: &str) -> String { // TODO: Replace "cars" in the string with "balloons". }