fix `doxygen` link

readme.md

@@ -47,7 +47,7 @@ RapidJSON is a header-only C++ library. Just copy the `include/rapidjson` folder
 
 RapidJSON uses following software as its dependencies:
 * [CMake](http://www.cmake.org) as a general build tool
-* (optional)[Doxygen](http://www.goxygen.org) to build documentation
+* (optional)[Doxygen](http://www.doxygen.org) to build documentation
 * (optional)[googletest](https://code.google.com/p/googletest/) for unit and performance testing
 
 To generate user documentation and run tests please proceed with the steps below: