Fix link from README to tutorial
Fix link from README to tutorial

This link wasn't showing up on github because of the linebreak between
the [] and () parts.  I also changed where the link points to since I
figured people reading the README would want to actually see the
rendered version of the tutorial rather than the source files for it.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script