fixed doxygen badge

README.md

@@ -2,7 +2,7 @@
 
 [![Build Status](https://travis-ci.org/nlohmann/json.png?branch=master)](https://travis-ci.org/nlohmann/json)
 [![Coverage Status](https://img.shields.io/coveralls/nlohmann/json.svg)](https://coveralls.io/r/nlohmann/json)
-[![Documentation Status](http://nlohmann.github.io/json)](https://img.shields.io/badge/documentation-doxygen-blue.svg)
+[![Documentation Status](https://img.shields.io/badge/documentation-doxygen-blue.svg)](http://nlohmann.github.io/json)
 [![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/nlohmann/json/master/LICENSE.MIT)
 [![Github Issues](https://img.shields.io/github/issues/nlohmann/json.svg)](http://github.com/nlohmann/json/issues)