Update styling-the-navigator.md

docs/styling-the-navigator.md

   navBarBackgroundColor: '#f7f7f7', // change the background color of the nav bar (remembered across pushes)
   navBarCustomView: 'example.CustomTopBar', // registered component name
   navBarComponentAlignment: 'center', // center/fill
+  navBarCustomViewInitialProps: {}, // Serializable JSON passed as props
   navBarButtonColor: '#007aff', // Change color of nav bar buttons (eg. the back button) (remembered across pushes)
 
   navBarHidden: false, // make the nav bar hidden