doc: enchane string3 exercise hint

rustlings-macros/info.toml

@@ -498,7 +498,11 @@ some of them:
 https://doc.rust-lang.org/std/string/struct.String.html#method.trim
 
 For the `compose_me` method: You can either use the `format!` macro, or convert
-the string slice into an owned string, which you can then freely extend."""
+the string slice into an owned string, which you can then freely extend.
+
+For the `replace_me` method: You can reference:
+https://doc.rust-lang.org/std/string/struct.String.html#method.replace
+"""
 
 [[exercises]]
 name = "strings4"