Change header for download page

follow up from https://github.com/LLK/scratch-www/pull/1475#issuecomment-326034683

src/views/download/l10n.json

 {
-  "download.title": "Scratch 2 Offline Editor",
+  "download.title": "Scratch 2.0 Offline Editor",
   "download.intro": "You can install the Scratch 2.0 editor to work on projects without an internet connection. This version will work on Mac, Windows, and some versions of Linux (32 bit).",
   "download.introMac": "<b>Note for Mac Users:</b> the latest version of Scratch 2.0 Offline requires Adobe Air 20. To upgrade to Adobe Air 20 manually, go <a href=\"https://get.adobe.com/air/\">here</a>.",
   "download.installation": "Installation",