Add trailing slash to `layouts` in readme.

This is to bring the readme up to parity with what was suggested in Automattic/underscores.me#24, meaning that files have their extensions and folders have a trailing slash. The only thing that needed to change was the reference to the layouts folder.

P.S. Sorry for all the incremental changes to the readme! It’s kind of nitpicky stuff, I know, but consistency is useful.