Update Buttons section of README

Was referencing `$button-style` hook that was removed in ca2b6170a3ee399824d254e52a021d8d72cf9fe3

Looking at the differences between `base/extends/_button.scss` and `base/_button.scss`, I assumed the extend is where default button styles should go.  The latter file seems intended for reasonable defaults / resets instead of styles.  Happy to update the text if this isn't the case.