Update install.haml

```bash
$ compass create myproject
```

Outputs

```
directory myproject/
directory myproject/sass/
directory myproject/stylesheets/
   create myproject/config.rb
   create myproject/sass/screen.scss
   create myproject/sass/print.scss
   create myproject/sass/ie.scss
   create myproject/stylesheets/print.css
   create myproject/stylesheets/screen.css
   create myproject/stylesheets/ie.css
```

So, the install guide should not read that `css` will be the default CSS output directory when in fact `stylesheets` is the default CSS output directory.