fix docs code block for `makeClassyPrisms`

The second code block in documentation for `makeClassyPrisms` doesn't render correctly. See e.g. [hackage docs](http://hackage.haskell.org/package/lens-4.5/docs/Control-Lens-TH.html#v:makeClassyPrisms).
Merge pull request #497 from benjaminfjones/patch-1

fix docs code block for `makeClassyPrisms`