Update package.json pointer to the new repository

Improve documentation of javascript npm package now that we're in a new repository
grpc/grpc-web#1241 
protocolbuffers/protobuf#9874

Author: samhowes

package.json

@@ -20,7 +20,7 @@
   },
   "repository": {
     "type": "git",
-    "url": "https://github.com/protocolbuffers/protobuf/tree/main/js"
+    "url": "https://github.com/protocolbuffers/protobuf-javascript"
   },
   "author": "Google Protocol Buffers Team",
   "license": "BSD-3-Clause"