fixes some typos in layered equations docs (#175)
fixes some typos in layered equations docs (#175)

* fixes some typos

Also, the topographic PV was both in `q_n` and in `Q_n`; now it's only part of `Q_n`.

* some more typos

also changes `\hat` --> `\widehat` which renders much better

rabernat/pyqg,pyqg/pyqg