change LoggingDriver to LogDriver

[chenhengqi]

Hello Brian,

I found this project from Docker docs, it's a good starting point for us to implement our own log driver plugin.
But it seems the LoggingDriver should be LogDriver to get it work.

docker: Error response from daemon: logger: no log driver named 'foobar' is registered.
See 'docker run --help'.

I found the related commit:
moby/moby@27bd684

const extName = "LogDriver"

Hope this PR is useful for those who find this repo.