[v2] Fix migration topBar fontSize example on docs (#3686)

On *Migration from V1 › Options* the `navBarTextFontSize` to topBar title fontSize example was set to 'red'. This PR fixes it to the default value, 18.

docs/docs/options-migration.md

 ```js
 topBar: {
   title: {
-    fontSize: 'red'
+    fontSize: 18
   }
 }
 ```
 * navBarSubTitleTextCentered
 * collapsingToolBarImage
 * collapsingToolBarCollapsedColor
-* navBarTextFontBold
+* navBarTextFontBold