Update chromedriver brew install instructions in contributing doc (#2…

…1251)

`brew cask install` was recently deprecated in favor of `brew install --cask`

contributing.md

@@ -18,7 +18,7 @@ To contribute to [our examples](examples), take a look at the [“Adding example
 
 Make sure you have `chromedriver` installed for your Chrome version. You can install it with
 
-- `brew cask install chromedriver` on Mac OS X
+- `brew install --cask chromedriver` on Mac OS X
 - `chocolatey install chromedriver` on Windows
 - Or manually download the version that matches your installed chrome version (if there's no match, download a version under it, but not above) from the [chromedriver repo](https://chromedriver.storage.googleapis.com/index.html) and add the binary to `<next-repo>/node_modules/.bin`