Readme Usage Typo

[mightym]

https://github.com/callemall/material-ui#usage
I guess it should be:

  getChildContext: function() {
    return {
      muiTheme: ThemeManager.getCurrentTheme()
    }
  }

[Zadielerick]

#856

[hai-cea]

Thanks @Zadielerick @mightym