[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

@@ -17,7 +17,7 @@ Title font size
 ```js
 topBar: {
   title: {
-    fontSize: 'red'
+    fontSize: 18
   }
 }
 ```
@@ -582,4 +582,4 @@ topBar: {
 * navBarSubTitleTextCentered
 * collapsingToolBarImage
 * collapsingToolBarCollapsedColor
-* navBarTextFontBold
+* navBarTextFontBold