Update README.md

Author: TheMuellenator

README.md

@@ -158,7 +158,8 @@
 * [DartPad For in Loop Mini Challenge](https://dartpad.dartlang.org/88fd2dd04c921892515bcd0567ef1636)
 * [Cupertino (iOS) Style Widgets](https://flutter.dev/docs/development/ui/widgets/cupertino)
 * [CupertinoPicker Class Documentation](https://docs.flutter.io/flutter/cupertino/CupertinoPicker-class.html)
-* [Bitcoin Average API Docs on Ticker Data](https://apiv2.bitcoinaverage.com/#ticker-data-per-symbol)
+* [Register for a coinapi.io API key](https://www.coinapi.io/Pricing)
+* [Coin API Docs](https://docs.coinapi.io/)
 * [Bronze Challenge Start Code](https://github.com/londonappbrewery/bitcoin-flutter-challenge-start)
 * [Silver Challenge Starter Code](https://github.com/londonappbrewery/bitcoin-flutter-silver-start)
 * [Gold Challenge Starter Code](https://github.com/londonappbrewery/bitcoin-flutter-gold-start)