Fixed typo in the exercieses README.md

Author: DerWario

exercises/README.md

@@ -9,7 +9,7 @@
 | vecs                   | §8.1                |
 | move_semantics         | §4.1-2              |
 | structs                | §5.1, §5.3          |
-| enums                  | §6, §18.3           |
+| enums                  | §6, §19.3           |
 | strings                | §8.2                |
 | modules                | §7                  |
 | hashmaps               | §8.3                |