Update folder shared link README.markdown

![1 ruby irontweet rb ruby 2014-03-21 13-57-43 2014-03-21 13-57-46](https://f.cloud.github.com/assets/1712162/2487919/a78e49a4-b13b-11e3-8deb-8105ee24b894.png)
cannot use hash syntax to access the shared link for url
works
p folder.shared_link.url # https://www.box.com/s/d6de3224958c1755412

looking forward to BOXDEV!
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