Merge pull request #795 from lauralindzey/docs/section-mapping

docs: Update exercise to chapter mapping for HashMap

exercises/README.md

@@ -10,7 +10,7 @@
 | structs                | §5.1          |
 | enums                  | §6            |
 | modules                | §7.2          |
-| collections            | §8.1          |
+| collections            | §8.1, §8.3    |
 | strings                | §8.2          |
 | error_handling         | §9            |
 | generics               | §10           |

exercises/collections/README.md

@@ -20,3 +20,4 @@ structures that are used very often in Rust programs:
 ## Further information
 
 - [Storing Lists of Values with Vectors](https://doc.rust-lang.org/stable/book/ch08-01-vectors.html)
+- [Storing Keys with Associated Values in Hash Maps](https://doc.rust-lang.org/book/ch08-03-hash-maps.html)