Fix app icon link in README.md (#1311)

Author: jwoglom

README.md

@@ -1,6 +1,6 @@
 # Loop for iOS
 
-![App Icon](/Loop/Assets.xcassets/AppIcon.appiconset/Icon-Small-40%402x.png?raw=true)
+![App Icon](/Loop/DefaultAssets.xcassets/AppIcon.appiconset/Icon-Small-40%402x.png?raw=true)
 
 [![Build Status](https://travis-ci.org/LoopKit/Loop.svg?branch=master)](https://travis-ci.org/LoopKit/Loop)
 [![Join the chat at https://loop.zulipchat.com](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://loop.zulipchat.com)