fix(iterators2): Remove reference to missing iterators2.rs
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-- iterators 1 and 2 are indeed missing. Iterator 3 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. iterators3.rs is a bit challenging so we're leaving space for some exercises to lead up to it!
