docs: Added link to HttpApi from "Event source types" (#1649)

Author: justinmk3

versions/2016-10-31.md

@@ -430,6 +430,7 @@ Properties:
  - [DynamoDB](#dynamodb)
  - [SQS](#sqs)
  - [Api](#api)
+ - [HttpApi](#httpapi)
  - [Schedule](#schedule)
  - [CloudWatchEvent](#cloudwatchevent)
  - [EventBridgeRule](#eventbridgerule)