docs: remove reference to sync.Locker

[ldez]

The interface sync.Locker is:

type Locker interface {
	Lock()
	Unlock()
}

https://github.com/golang/go/blob/5a18e79687dea15680ff5f799b549fa0efd0cad9/src/sync/mutex.go#L41-L45

But the method signatures of Flock are:

type Foo interface {
	Lock() error
	Unlock() error
}

So to implement sync.Locker interface, a wrapper should be used.

Example:

type Wrapper struct {
	f *Flock
}

func NewWrapper(path string) *Wrapper {
	return &Wrapper{f: flock.New(path)}
}

func (w Wrapper) Lock() {
	err := w.f.Lock()
	if err != nil {
		panic(err)
	}
}

func (w Wrapper) Unlock() {
	err := w.f.Unlock()
	if err != nil {
		panic(err)
	}
}

Fixes #45