docs: Fixes CONTRIBUTING.md referencing old flutter format command

[ahmetakil]

Description

The command flutter format . has been updated as dart format .
This PR updates the CONTRIBUTING.md doc to reflect this change

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.