Add plurals to grid demo

[ergophobe]

It would be nice to

• list which units are possible for grids (twelfth, sixth, quarter, third, half)
• how to write plurals (i.e. it's large--three-quarters, not large--three-quarter).

It would only take a couple of lines, and currently you have to use trial and error or go the csswizardry docs to verify such a simple thing.