docs: update the example code in `docs/share_plus/usage.mdx`

[albinpk]

Description

Fixed the issue in the example code (docs/share_plus/usage.mdx)

Checklist

• [X] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [X] I titled the PR using Conventional Commits.
• [X] I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• [X] All existing and new tests are passing.
• [X] 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).
• [X] No, this is not a breaking change.