docs: Fixed links to Proguard manual. Bug: 12713887

The SourceForge manual cannot be linked to sections directly because
of JavaScript and HTML frames. I used the MIT links from the bug
report instead.

Review: http://quixote.mtv.corp.google.com:8001/tools/help/proguard.html

Change-Id: I1be210d392d78a7560a89f421074ba6d99499da0