Fix the link from Index to Setup Guide (GH-229)
Fix the link from Index to Setup Guide (GH-229)

A link to setup.rst was missing its :ref: prefix.
ezio-melotti/devguide,jython/devguide,jython/devguide,jeff5/jython-devguide,jython/devguide,jeff5/jython-devguide,jeff5/jython-devguide