Correct filename to that which exists in master branch of the repo.

Brew says: 
```
$ brew install haskell-platform
Error: No available formula for haskell-platform
We no longer package haskell-platform. Consider installing ghc
and cabal-install instead:
  brew install ghc cabal-install

A binary installer is available:
  https://www.haskell.org/platform/mac.html
```

Change "simply" to "simple".