added missing ',' on example code (#1147)

docs/adding-buttons-to-the-navigator.md

         testID: 'e2e_rules', // optional, used to locate this view in end-to-end tests
         disabled: true, // optional, used to disable the button (appears faded and doesn't interact)
         disableIconTint: true, // optional, by default the image colors are overridden and tinted to navBarButtonColor, set to true to keep the original image colors
-        showAsAction: 'ifRoom' // optional, Android only. Control how the button is displayed in the Toolbar. Accepted valued: 'ifRoom' (default) - Show this item as a button in an Action Bar if the system decides there is room for it. 'always' - Always show this item as a button in an Action Bar. 'withText' - When this item is in the action bar, always show it with a text label even if it also has an icon specified. 'never' - Never show this item as a button in an Action Bar.
+        showAsAction: 'ifRoom', // optional, Android only. Control how the button is displayed in the Toolbar. Accepted valued: 'ifRoom' (default) - Show this item as a button in an Action Bar if the system decides there is room for it. 'always' - Always show this item as a button in an Action Bar. 'withText' - When this item is in the action bar, always show it with a text label even if it also has an icon specified. 'never' - Never show this item as a button in an Action Bar.
         buttonColor: 'blue', // Set color for the button (can also be used in setButtons function to set different button style programatically)
         buttonFontSize: 14, // Set font size for the button (can also be used in setButtons function to set different button style programatically)
         buttonFontWeight: '600', // Set font weight for the button (can also be used in setButtons function to set different button style programatically)
   contextualMenuBackgroundColor: '#00adf5',
   contextualMenuButtonsColor: '#ffffff'
 };
-```
+```