fix incorrect store usage

[zloybest]

I got error when try change navigationStateKey to anything not equal navigation.
With this fix it works.

[booboothefool]

@zloybest This was driving me nuts! Also, it seems that when you use navigation like in the readme, it replaces the usual this.props.navigation. As a new user of this library trying to toggle a drawer, I was wondering why this.props.navigation.getNavigator('...') didn't exist (what you get for when you search toggling a drawer programmatically). Since navigation gets replaced, had to do navigator.navigationContext.getNavigator('main').toggleDrawer(). Not the best example for newbies...

And this is the error @zloybest is talking about: