Add plugin website to manifest.

plugin.json

@@ -7,7 +7,7 @@
   "author": "VCV",
   "authorEmail": "contact@vcvrack.com",
   "authorUrl": "https://vcvrack.com/",
-  "pluginUrl": "",
+  "pluginUrl": "https://vcvrack.com/Prototype.html",
   "manualUrl": "https://github.com/VCVRack/VCV-Prototype/blob/master/README.md",
   "sourceUrl": "https://github.com/VCVRack/VCV-Prototype",
   "donateUrl": "",