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
(cherry picked from commit f75a50d9e249986bb3da1f969b286f82002f33a6)
Fix URL link in json example.

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

Change-Id: I808290c42b633e74dfca896b4fd4124a6efd9dfe
(cherry picked from commit 17ec26cf37b90398c0872867eaa59f2930322676)