docs: fix homepage 404 in npm package (#1458)

Based on this comment I think this is the correct URL.
#1210 (comment)

Co-authored-by: Alexander Fenster <fenster@google.com>

package.json

@@ -7,7 +7,7 @@
   "license": "BSD-3-Clause",
   "repository": "protobufjs/protobuf.js",
   "bugs": "https://github.com/dcodeIO/protobuf.js/issues",
-  "homepage": "http://dcode.io/protobuf.js",
+  "homepage": "https://protobufjs.github.io/protobuf.js/",
   "keywords": [
     "protobuf",
     "protocol-buffers",