Update styling-the-navigator.md

docs/styling-the-navigator.md

 {
   // Common
   navBarTextColor: '#000000', // change the text color of the title (remembered across pushes)
+  navBarTextFontSize: 18, // change the font size of the title
   navBarTextFontFamily: 'font-name', // Changes the title font
   navBarBackgroundColor: '#f7f7f7', // change the background color of the nav bar (remembered across pushes)