Update README.md

[eoeefosa]

What does this change?

flutter pub run depreciated
"Dart run" is used instead

Fixes #<issue_number_goes_here> 🎯
flutter pub run depreciated

Type of change

-$ flutter packages pub run build_runner build
$ Dart run build_runner build

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open a GitHub issue as a bug/feature request before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
  • Ensure the tests (melos run unit:test)
  • Ensure the analyzer and formatter pass (melos run format to automatically apply formatting)
• Appropriate docs were updated (if necessary)