Fix docs typo in Serializers Validators (#9563)

docs/api-guide/serializers.md

@@ -233,7 +233,7 @@ Serializer classes can also include reusable validators that are applied to the
 
     class EventSerializer(serializers.Serializer):
         name = serializers.CharField()
-        room_number = serializers.IntegerField(choices=[101, 102, 103, 201])
+        room_number = serializers.ChoiceField(choices=[101, 102, 103, 201])
         date = serializers.DateField()
 
         class Meta: