fs: rename event to eventType in fs.watch listener

The name 'event' for the argument of the listener in
fs.watch was confusing considering FSWatcher also had
events. This changes the name of the argument to
eventType.

Fixes: https://github.com/nodejs/node/issues/7504
PR-URL: https://github.com/nodejs/node/pull/7506


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>