docs: correct aggregation function list in aggregate.md (#6002)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: prql-bot

web/book/src/reference/stdlib/transforms/aggregate.md

@@ -14,7 +14,7 @@ aggregate {expression or assign operations}
 <!-- prettier-ignore -->
 > [!NOTE]
 > Currently, all declared aggregation functions are `min`, `max`,
-> `count`, `average`, `stddev`, `avg` and `sum`. We are in the
+> `count`, `count_distinct`, `average`, `stddev` and `sum`. We are in the
 > process of filling out [std lib](../).
 
 ## Examples