docs: add link to docs about iter_mut and map

Author: magnusrodseth

exercises/vecs/README.md

@@ -13,3 +13,5 @@ the other useful data structure, hash maps, later.
 ## Further information
 
 - [Storing Lists of Values with Vectors](https://doc.rust-lang.org/stable/book/ch08-01-vectors.html)
+- [`iter_mut`](https://doc.rust-lang.org/std/primitive.slice.html#method.iter_mut)
+- [`map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.map)