Fix JSON example.

It seems that previous versions of GWT allowed entry point classes not
implementing EntryPoint but having the onModuleLoad method, but 2.7.0
disallows it for production and SDM, classic dev mode works though.

Change-Id: I902d8193b59c580f1d5ed52f73e2f232d0baaabf
Fix URL link in json example.

Yahoo link does not exist any more.
Formatted json file.

Change-Id: I808290c42b633e74dfca896b4fd4124a6efd9dfe