gtk: Fix the Gitter badge
-# gtk [![Build Status](https://travis-ci.org/gtk-rs/gtk.png?branch=master)](https://travis-ci.org/gtk-rs/gtk) [![Build status](https://ci.appveyor.com/api/projects/status/5mot32ipr12iocw0?svg=true)](https://ci.appveyor.com/project/GuillaumeGomez/gtk) [![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/gtk-rs/gtk)
+# gtk [![Build Status](https://travis-ci.org/gtk-rs/gtk.png?branch=master)](https://travis-ci.org/gtk-rs/gtk) [![Build status](https://ci.appveyor.com/api/projects/status/5mot32ipr12iocw0?svg=true)](https://ci.appveyor.com/project/GuillaumeGomez/gtk) [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/gtk-rs/gtk)
[Project site](http://gtk-rs.org/) | [Online documentation](http://gtk-rs.org/docs/)
