Add navBarSubtitleFontFamily to docs (#1395)

docs/styling-the-navigator.md

   statusBarTextColorScheme: 'dark', // text color of status bar, 'dark' / 'light' (remembered across pushes)
   statusBarTextColorSchemeSingleScreen: 'light' // same as statusBarTextColorScheme but does NOT remember across pushes
   navBarSubtitleColor: 'red', // subtitle color
+  navBarSubtitleFontFamily: 'font-name', // subtitle font
   screenBackgroundColor: 'white', // Default screen color, visible before the actual react view is rendered
   orientation: 'portrait' // Sets a specific orientation to a modal and all screens pushed to it. Default: 'auto'. Supported values: 'auto', 'landscape', 'portrait'