fix the javadocs badge
The current link redirects to javadoc.io with this error message:
> Artifact io:etcd does not exist.
[![License](https://img.shields.io/badge/Licence-Apache%202.0-blue.svg?style=flat-square)](http://www.apache.org/licenses/LICENSE-2.0.html)
[![Maven Central](https://img.shields.io/maven-central/v/io.etcd/jetcd-core.svg?style=flat-square)](https://search.maven.org/#search%7Cga%7C1%7Cio.etcd)
[![GitHub release](https://img.shields.io/github/release/etcd-io/jetcd.svg?style=flat-square)](https://github.com/etcd-io/jetcd/releases)
-[![Javadocs](http://www.javadoc.io/badge/io/etcd/jetcd-core.svg)](http://www.javadoc.io/doc/io/etcd/jetcd-core)
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+[![Javadocs](http://www.javadoc.io/badge/io/etcd/jetcd-core.svg)](https://javadoc.io/doc/io.etcd/jetcd-core)
jetcd is the official java client for [etcd](https://github.com/etcd-io/etcd) v3.
