Fix download footnotes links (#401)

Fixes the links in downloads -> older releases blocks in
`download/_build-platform.html`. The links used markdown syntax in an
html file, and hence would not render correctly.

Author: natikgadzhi

download/_build-platform.html

@@ -67,5 +67,5 @@
     </tbody>
 </table>
 
-<sup>1</sup> Swift {{ site.data.builds.swift_releases.last.name }} is available as part of [{{ include.platform.xcode }}](https://itunes.apple.com/app/xcode/id497799835).<br>
-<sup>2</sup> Swift {{ site.data.builds.swift_releases.last.name }} Windows 10 toolchain is provided by [Saleem Abdulrasool](https://github.com/compnerd). Saleem is the platform champion for the Windows port of Swift and this is an official build from the Swift project. <br><br>
+<sup>1</sup> Swift {{ site.data.builds.swift_releases.last.name }} is available as part of <a href="https://itunes.apple.com/app/xcode/id497799835">{{ include.platform.xcode }}</a>.<br>
+<sup>2</sup> Swift {{ site.data.builds.swift_releases.last.name }} Windows 10 toolchain is provided by <a href="https://github.com/compnerd">Saleem Abdulrasool</a>. Saleem is the platform champion for the Windows port of Swift and this is an official build from the Swift project. <br><br>