auto-hash-ref/explicit: Add an example to the comment.

utils.rkt

@@ -17,6 +17,11 @@
 ;;; Given a (HashTable Symbol a) and a sequence of symbols, binds
 ;;; these symbols to the values they are associated to in the hash
 ;;; table, then puts the body in the context of these bindings.
+;;;
+;;; > (let ([ht #hash((a . 1) (b . 2))])
+;;;     (auto-hash-ref/explicit (ht a b) (+ a (* 2 b))))
+;;; 5
+;;;
 (define-syntax (auto-hash-ref/explicit stx)
   (syntax-parse stx
     [(_ (ht:id xs:id ...) body:expr)