Update tutorial_polymers.rst to read the notebook
Update tutorial_polymers.rst to read the notebook

Keeping the .rst file like this means we don't have to maintain two
copies of the same thing.

ctk3b/mbuild,summeraz/mbuild,summeraz/mbuild,tcmoore3/mbuild,tcmoore3/mbuild,iModels/mbuild,iModels/mbuild,ctk3b/mbuild