[ FLOC-1010] Improvements to the release process documentation

Fixes https://clusterhq.atlassian.net/browse/FLOC-1010
No, it's not a variable. It just signifies that the file you downloaded will have a specific version. And you should use that file, whatever it is. eg
 * https://storage.googleapis.com/clusterhq-vagrant-buildbot/tutorial/flocker-tutorial-0.3.2-337-g2b808b9.box