mediaQueries example and text change

Changes:

1. Updated the main code example to use the same grid keys and breakpoints as defined via the [Pure grids docs](http://purecss.io/grids/#pure-responsive-grids) (i.e., I changed `med`, `lrg` to `md` and `lg` respectively); this will save anyone trouble, for example, if copy/pasting from README and they've already been using grids HTML as defined via main PURE docs.
1. Updated `options.mediaQueries` option description to clarify how the `key` works in terms of generating class names (using `FOO` to make sure it's clear that the key is used to generate class names).