colour scales tutorial, minor issues

https://github.com/ricklupton/floweaver/blob/21abc95/docs/tutorials/colour-scales.ipynb

Line 173, "emjoys"

Lines 194-195, received this warning (using Binder):
> /srv/conda/envs/notebook/lib/python3.11/site-packages/floweaver/color_scales.py:128: RuntimeWarning: invalid value encountered in scalar divide
  value /= measures[self.intensity]