fix artifact_id in maven central badge

Author: Mahmoud Ben Hassine

README.md

@@ -2,7 +2,7 @@
 
 [![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/benas/UnixStream?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![MIT license](http://img.shields.io/badge/license-MIT-brightgreen.svg?style=flat)](http://opensource.org/licenses/MIT)
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.github.benas/UnixStream/badge.svg?style=flat)](http://search.maven.org/#artifactdetails|io.github.benas|unix-stream|0.3|)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.github.benas/unix-stream/badge.svg?style=flat)](http://search.maven.org/#artifactdetails|io.github.benas|unix-stream|0.3|)
 [![Build Status](https://travis-ci.org/benas/UnixStream.svg?branch=master)](https://travis-ci.org/benas/UnixStream)
 
 UnixStream is an extension of the Java 8 Stream API to process data pipelines the Unix way.