add sdk version constraint to example app

[mjkim]

when I build the example app, below error are occurred.

pubspec.yaml has no lower-bound SDK constraint.
You should edit pubspec.yaml to contain an SDK constraint:

environment:
  sdk: '>=2.10.0 <3.0.0'

See https://dart.dev/go/sdk-constraint

and, this error can be cured by add SDK constraint.
(tested)