Use stealTools.optimize in the Progressive Loading guide

[matthewp]

https://github.com/stealjs/steal/issues/1261 showed the need to move away from stealTools.build as the recommended way to build projects. Updating the progressive loading guide to use optimize instead would have prevented this (although the bug would still have been there, of course).

If we change the guide we need to figure out what to do about the "Optimized builds" guide. Maybe it's unnecessary?