doc: update book links

Author: manyinsects

exercises/README.md

@@ -5,20 +5,22 @@
 | variables              | §3.1                |
 | functions              | §3.3                |
 | if                     | §3.5                |
-| move_semantics         | §4.1, §4.2          |
 | primitive_types        | §3.2, §4.3          |
+| vecs                   | §8.1                |
+| move_semantics         | §4.1, §4.2          |
 | structs                | §5.1, §5.3          |
 | enums                  | §6, §18.3           |
-| modules                | §7                  |
-| collections            | §8.1, §8.3          |
 | strings                | §8.2                |
+| modules                | §7                  |
+| hashmaps               | §8.3                |
+| options                | §10.1               |
 | error_handling         | §9                  |
 | generics               | §10                 |
-| option                 | §10.1               |
 | traits                 | §10.2               |
 | tests                  | §11.1               |
+| lifetimes              | §10.3               |
 | standard_library_types | §13.2, §15.1, §16.3 |
-| threads                | §16.1               |
+| threads                | §16.1, §16.2, §16.3 |
 | macros                 | §19.6               |
 | clippy                 | n/a                 |
 | conversions            | n/a                 |