documentation fix

Little documentation fix (the example code was not compiling before).
I'll take this as a chance to ask you: Why is ```levelHooks``` private?

If I try to implement your example in a package that is not in logrus this does not work:

```go
l := &logrus.Logger{
    Out: os.Stderr,
    Formatter: new(JSONFormatter),
    Hooks: make(levelHooks), // it does not compile because levelHooks is private
    Level: logrus.DebugLevel,
}
```