fix a typo

Author: OccupyMars2025

src/macros-by-example.md

@@ -166,7 +166,7 @@ The repetition operators are:
 
 - `*` — indicates any number of repetitions.
 - `+` — indicates any number but at least one.
-- `?` — indicates an optional fragment with zero or one occurrences.
+- `?` — indicates an optional fragment with zero or one occurrence.
 
 Since `?` represents at most one occurrence, it cannot be used with a
 separator.