doc: Fix link to documentation.

Author: freezy

VisualPinball.Engine.Mpf.Unity/package.json

@@ -16,7 +16,7 @@
   "contributors": [
     "jsm174 <[email protected]>"
   ],
-  "documentationUrl": "https://docs.visualpinball.org/",
+  "documentationUrl": "https://docs.visualpinball.org/plugins/mpf/",
   "changelogUrl": "https://docs.visualpinball.org/CHANGELOG.html",
   "licensesUrl": "https://github.com/VisualPinball/VisualPinball.Unity.Hdrp/blob/master/LICENSE",
   "repository": {