Fix example: logrus.AddHook() args + add deferred flush

[angristan]

The levels have to be set earlier, otherwise the example can't run:

» go run main.go
# command-line-arguments
./main.go:27:13: too many arguments in call to logrus.AddHook
	have (*"github.com/onrik/logrus/sentry".Hook, logrus.Level, logrus.Level, logrus.Level)
	want (logrus.Hook)

Also, sentry-go flushes event asynchronously, so if we don't wait after the final log.Error(), the program will be terminated before the event is sent. I added a defer sentryHook.Flush() otherwise the example doesn't work.