Update MIGRATION_GUIDE_v3.md

Wrap `setConfigProperty` calls in `map.on('style.load')` handlers

MIGRATION_GUIDE_v3.md

@@ -83,15 +83,19 @@ With Mapbox Standard, we are also introducing a new paradigm for how to interact
 * The Mapbox Standard style features 4 light presets: "Day", "Dusk", "Dawn", and "Night". The style light preset can be changed from the default, "Day", to another preset with a single line of code:
 
 ```js
-map.setConfigProperty('basemap', 'lightPreset', 'dusk');
+map.on('style.load', () => {
+    map.setConfigProperty('basemap', 'lightPreset', 'dusk');
+});
 ```
 
 Changing the light preset will alter the colors and shadows on your map to reflect the time of day. For more information, refer to the [Lighting API](#lighting-api) section.
 
 Similarly, you can set other configuration properties on the Standard style such as showing POIs, place labels, or specific fonts:
 
 ```js
-map.setConfigProperty('basemap', 'showPointOfInterestLabels', false);
+map.on('style.load', () => {
+    map.setConfigProperty('basemap', 'showPointOfInterestLabels', false);
+});
 ```
 
 The Standard style offers 6 configuration properties for developers to change when they import it into their own style: