renamed webp-quicklook to webpquicklook

As stated by @rolandwalker in #3344, we should:

> try to avoid the case where a Cask name differs only by the placement of a hyphen.

However, it seems we forgot to change these (the other being [`emin-webpquicklook.rb`](https://github.com/caskroom/homebrew-cask/blob/1fc5bebf3f0aebd117e4a3065f85b5afec263f04/Casks/emin-webpquicklook.rb)), even though they were part of the rule’s mention. This PR addresses that.