Fix 1 occurrence of for/fold-with-conditional-body-to-unless-keyword

This `for/fold` loop can be simplified by using the `#:unless` keyword.

Author: resyntax-ci[bot]

typed-racket-lib/typed-racket/private/parse-type.rkt

@@ -1507,10 +1507,9 @@
   ;; Merge all the non-duplicate entries from the parent types
   (define (merge-clause parent-clause clause)
     (for/fold ([clause clause])
-              ([(k v) (in-dict parent-clause)])
-      (if (dict-has-key? clause k)
-          clause
-          (dict-set clause k v))))
+              ([(k v) (in-dict parent-clause)]
+               #:unless (dict-has-key? clause k))
+      (dict-set clause k v)))
 
   (define (match-parent-type parent-type)
     (define resolved (resolve parent-type))