Fix incorrect book chapter number

aCactusOrSomething · aCactusOrSomething · commit 3a2fe2c39472 · 2025-07-18T13:02:56.000-05:00
diff --git a/exercises/README.md b/exercises/README.md
@@ -22,6 +22,6 @@
 | iterators              | §13.2-4             |
 | smart_pointers         | §15, §16.3          |
 | threads                | §16.1-3             |
-| macros                 | §19.5               |
+| macros                 | §20.5               |
 | clippy                 | §21.4               |
 | conversions            | n/a                 |
