Auto merge of #165 - gushroom:fix-outdated-links, r=komaeda

fixed outdated links

The links had the 2018 book in them and they were outdated.

Author: bors

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/2018-edition/ch16-03-shared-state.html) of the Rust Book.
+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/2018-edition/ch13-02-iterators.html) of the Rust Book and the [Iterator documentation](https://doc.rust-lang.org/stable/std/iter/trait.Iterator.htmlj).
+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!
 

exercises/threads/README.md

@@ -1 +1 @@
-For this exercise check out the [Dining Philosophers example](https://doc.rust-lang.org/1.4.0/book/dining-philosophers.html) and the chapter [Concurrency](https://doc.rust-lang.org/book/2018-edition/ch16-01-threads.html) of the Rust Book.
+For this exercise check out the [Dining Philosophers example](https://doc.rust-lang.org/1.4.0/book/dining-philosophers.html) and the chapter [Concurrency](https://doc.rust-lang.org/book/ch16-01-threads.html) of the Rust Book.