Fix code coverage link

Author: akarnokd

README.md

@@ -1,7 +1,7 @@
 # RxJava: Reactive Extensions for the JVM
 
 <a href='https://travis-ci.org/ReactiveX/RxJava/builds'><img src='https://travis-ci.org/ReactiveX/RxJava.svg?branch=2.x'></a>
-[![codecov.io](http://codecov.io/github/ReactiveX/RxJava/coverage.svg?branch=2.x)](http://codecov.io/github/ReactiveX/RxJava?branch=2.x)
+[![codecov.io](http://codecov.io/github/ReactiveX/RxJava/coverage.svg?branch=2.x)](https://codecov.io/gh/ReactiveX/RxJava/branch/2.x)
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.reactivex.rxjava2/rxjava/badge.svg)](https://maven-badges.herokuapp.com/maven-central/io.reactivex.rxjava2/rxjava)
 
 RxJava is a Java VM implementation of [Reactive Extensions](http://reactivex.io): a library for composing asynchronous and event-based programs by using observable sequences.