Fix README typo

I was wondering why snippets under `~/.atom/snippet.cson` weren't available. 

```coffee
  getUserSnippetsPath: ->
    userSnippetsPath = CSON.resolve(path.join(atom.getConfigDirPath(), 'snippets'))
    userSnippetsPath ? path.join(atom.getConfigDirPath(), 'snippets.cson')
```