Wrong URL to Sphinx docs in README file

[jonancm]

The README file on the master branch as of commit a56f23a contains a link to the Sphinx docs at http://genba.github.io/viennagrid-python/, which is clearly the wrong URL (it should be http://jonancm.github.io/viennagrid-python/).

viennagrid-python/setup.py also specifies the wrong URL.