Fix links in README

Author: Kaelig Deloumeau-Prigent

README.md

@@ -4,7 +4,7 @@
 <img src="https://cloud.githubusercontent.com/assets/85783/15560157/af4a1abc-229d-11e6-9a3d-6c3f4b81220b.png" alt="Design System UI Kit" />
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-A [Sketch](https://www.sketchapp.com/) file of common design patterns from the [Lightning Design System](https://www.lightningdesignsystem.com).
+A <a href="https://www.sketchapp.com/">Sketch</a> file of common design patterns from the <a href="https://www.lightningdesignsystem.com">Lightning Design System</a>.
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 <h3 align="center"><a href="https://github.com/salesforce-ux/design-system-ui-kit/archive/master.zip">» Download the latest version «</a></h3>
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