update maven central URL in README.md
update maven central URL in README.md

Update the "maven search url" http://search.maven.org/#search%7Cga%7C1%7Cio.reactivex.rxjavafx instead of http://search.maven.org/#search%7Cga%7C1%7Cio.reactivex.rxjava-reactive-streams
thomasnield/RxJavaFX,ReactiveX/RxJavaFX,ReactiveX/RxJavaFX,thomasnield/RxJavaFX