Update Flutter Gallery link (#429)

gallery/README.md

@@ -4,4 +4,4 @@ https://github.com/flutter/gallery
 
 To see a compiled Gallery running in your browser, use this link: 
 
-https://flutter.github.io/gallery
+https://gallery.flutter.dev

web/samples_index/lib/src/samples.yaml

@@ -13,7 +13,7 @@ samples:
       - url: images/gallery5.png
         alt: Shrine app screnshot
     source: https://github.com/flutter/gallery
-    web: https://flutter.github.io/gallery
+    web: https://gallery.flutter.dev
     description: >
       A collection of material design widgets, behaviors, and vignettes
       implemented with Flutter.