Update pubspec.yaml.tmpl (flutter#12031)

Update to reflect flutter#11751 and flutter/website#663

Author: mit-mit

packages/flutter_tools/templates/create/pubspec.yaml.tmpl

@@ -37,17 +37,9 @@ flutter:
   # An image asset can refer to one or more resolution-specific "variants", see
   # https://flutter.io/assets-and-images/.
 
-  # To add assets from package dependencies, first ensure the asset
-  # is in the lib/ directory of the dependency. Then,
-  # refer to the asset with a path prefixed with
-  # `packages/PACKAGE_NAME/`. The `lib/` is implied, do not
-  # include `lib/` in the asset path.
-  #
-  # Here is an example:
-  #
-  # assets:
-  #  - packages/PACKAGE_NAME/path/to/asset
-
+  # For details regarding adding assets from package dependencies, see
+  # https://flutter.io/assets-and-images/
+  
   # To add custom fonts to your application, add a fonts section here,
   # in this "flutter" section. Each entry in this list should have a
   # "family" key with the font family name, and a "fonts" key with a