Fix alignment typo in README
Fix alignment typo in README

The readme states that a slide's `align` property takes two values, both of which can either by `flex-start` or `center` or `flex-end`, but then states that the default is `align="center middle"`, which should actually be `align="center center"` I think, so that change has been made. Also fixed `it's` to `its` typo on line 241.
lowsky/spectacle,waywaaard/spectacle,FormidableLabs/spectacle,lowsky/spectacle,FormidableLabs/spectacle,waywaaard/spectacle