Fix github light release link

The github releases page was wrong. This will help users to install light faster.

README.md

 ### Debian/Ubuntu
 
 Pre-built .deb files, for the latest Ubuntu release, can be downloaded
-from the [GitHub][Light] releases page.  If you want to build your own
+from the [GitHub](https://github.com/haikarainen/light/releases/) releases page.  If you want to build your own
 there is native support available in the GIT sources.  Clone and follow
 the development branch guidelines below followed by: