Update sdk_auto_install.rst

In the actual url pattern, you reference the page with only actual (latest) build. So everyone has to use it. If you try, to run a build, when new version of sdk is released, and you don't have the build in your local cache, build fails. (for example http://archive.apache.org/dist/flex/4.9.0/binaries/apache-flex-sdk-4.9.0-bin.zip doesn't exist). This fix make reference to repository, where're all versions of SDK, so you're not obligated to changed version every time new SDK is released.