Update install instruction in README.md (#1312)

Warning: Calling brew cask install is deprecated! Use brew install [--cask] instead.

Author: davidpanofsky

README.md

@@ -19,7 +19,7 @@ GraphQL IDE for better development workflows (GraphQL Subscriptions, interactive
 ## Installation
 
 ```sh
-$ brew cask install graphql-playground
+$ brew install --cask graphql-playground
 ```
 
 ## Features