Update README.md (#166)

s/twitch/source_gen :)

Author: matanlurey

README.md

@@ -5,7 +5,7 @@
     <img src="https://travis-ci.org/dart-lang/source_gen.svg?branch=master" alt="Build Status" />
   </a>
   <a href="https://pub.dartlang.org/packages/source_gen">
-    <img src="https://img.shields.io/pub/v/twitch.svg" alt="Pub Package Version" />
+    <img src="https://img.shields.io/pub/v/source_gen.svg" alt="Pub Package Version" />
   </a>
   <a href="https://gitter.im/dart-lang/source_gen">
     <img src="https://badges.gitter.im/dart-lang/source_gen.svg" alt="Join the chat on Gitter" />