Update broken link

resolves #93

Author: Sauermann

README.rst

@@ -112,7 +112,7 @@ contacts:
 .. _asyncio: https://docs.python.org/3/library/asyncio.html
 .. _venv: https://docs.python.org/3/library/venv.html
 .. _virtualenvwrapper: https://virtualenvwrapper.readthedocs.io/
-.. _libsodium: https://download.libsodium.org/doc/installation/index.html
+.. _libsodium: https://download.libsodium.org/doc/installation/
 
 .. |Travis| image:: https://travis-ci.org/saltyrtc/saltyrtc-server-python.svg?branch=master
    :target: https://travis-ci.org/saltyrtc/saltyrtc-server-python