reworded missing exercise explanation

Author: sjmann

exercises/standard_library_types/README.md

@@ -1,5 +1,5 @@
 For the Arc exercise check out the chapter [Shared-State Concurrency](https://doc.rust-lang.org/book/ch16-03-shared-state.html) of the Rust Book.
 
 For the Iterator exercise check out the chapters [Iterator](https://doc.rust-lang.org/book/ch13-02-iterators.html) of the Rust Book and the [Iterator documentation](https://doc.rust-lang.org/stable/std/iter/).
-Do not adjust your monitors-- iterators1.rs is indeed missing. iterators3.rs is a bit challenging so we're leaving space for some exercises to lead up to it!
+Do not adjust your monitors-- iterators1.rs is indeed missing. Iterators is a challenging topic, so we're leaving space for a simpler exercice!