Update index.dok

Added <Use an empty string (2 apostophes in a row) if you want to keep the current value.> for keeping the current value of {xmin,xmax,ymin,ymax} in <gnuplot.axis> documentation. Actually, I don't know how to write the <''>, since these are a command in the documentation..
Merge pull request #145 from Atcold/patch-1

Update index.dok