Correct the contract in Example 3 to use and/c.

Closes PR 13486.

collects/scribblings/guide/contracts-examples/3-test.rkt

@@ -9,4 +9,7 @@
   "dictionaries"
   (test-equal? "value for" 2 (value-for d 'b))
   (test-false "has?" (has? (rem d 'b) 'b))
-  (test-equal? "count" 3 (count d))))
+  (test-equal? "count" 3 (count d))
+  (test-case "contract check for put: symbol?"
+             (define d0 (initialize (flat-contract integer?) =))
+             (check-exn exn:fail:contract? (lambda () (put d0 "a" 2))))))

collects/scribblings/guide/contracts-examples/3.rkt

@@ -59,7 +59,7 @@
   ;; via initialize. We could do the same via a key? parameter
   ;; (exercise). add key k with value v to this dictionary 
   [put         (->d ([d dictionary?]
-                     [k (and symbol? (not-has? d))]
+                     [k (and/c symbol? (not-has? d))]
                      [v (dictionary-value? d)])
                     ()
                     [result dictionary?]