Make upgrade wizard download link be relative

Ensures that the prerelease upgrade wizard download link doesn’t point
to the non-prerelease version

plugins/tiddlywiki/upgrade/UpgradeWizard.tid

@@ -50,7 +50,7 @@ For help and support, visit [[the TiddlyWiki discussion forum|http://groups.goog
 
 version <<version>>
 
-//Your data will not leave your browser. <a href="http://tiddlywiki.com/upgrade.html" download="upgrade.html">Download</a> this upgrader to use it offline//
+//Your data will not leave your browser. <a href="#" download="upgrade.html">Download</a> this upgrader to use it offline//
 
 //If clicking the link doesn't work, right-click the link and save it that way.//