docs: Add missing exercise to book chapter mapping

Author: cteea

exercises/README.md

@@ -1,24 +1,24 @@
 # Exercise to Book Chapter mapping
 
-| Exercise               | Book Chapter |
-|------------------------|--------------|
-| variables              | §3.1          |
-| functions              | §3.3          |
-| if                     | §3.5          |
-| move_semantics         | §4.1          |
-| primitive_types        | §4.3          |
-| structs                | §5.1          |
-| enums                  | §6            |
-| modules                | §7            |
-| collections            | §8.1, §8.3    |
-| strings                | §8.2          |
-| error_handling         | §9            |
-| generics               | §10           |
-| option                 | §10.1         |
-| traits                 | §10.2         |
-| tests                  | §11.1         |
-| standard_library_types | §13.2         |
-| threads                | §16.1         |
-| macros                 | §19.6         |
-| clippy                 | n/a           |
-| conversions            | n/a           |
+| Exercise               | Book Chapter        |
+| ---------------------- | ------------------- |
+| variables              | §3.1                |
+| functions              | §3.3                |
+| if                     | §3.5                |
+| move_semantics         | §4.1, §4.2          |
+| primitive_types        | §3.2, §4.3          |
+| structs                | §5.1, §5.3          |
+| enums                  | §6, §18.3           |
+| modules                | §7                  |
+| collections            | §8.1, §8.3          |
+| strings                | §8.2                |
+| error_handling         | §9                  |
+| generics               | §10                 |
+| option                 | §10.1               |
+| traits                 | §10.2               |
+| tests                  | §11.1               |
+| standard_library_types | §13.2, §15.1, §16.3 |
+| threads                | §16.1               |
+| macros                 | §19.6               |
+| clippy                 | n/a                 |
+| conversions            | n/a                 |