docs(all): Fix example links on pub.dev

[vbuberen]

Description

Due to wrong links under repository tag in pubspec.yaml of every single package we have an issue where on pub.dev clicking on example/lib/main.dart link (here it is at the top of the page), which is generated by pub.dev itself, users were redirected to 404 page.
Found a solution here: dart-lang/pub-dev#3288

Related Issues

Closes #1861

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.