docs(share_plus): Fix supported platforms in README

[shatanikmahanty]

Updated file-sharing compatibility text for desktop platforms

Description

The readme file had misleading info stating file sharing is not supported on Linux and Windows. But on Windows file sharing was working. This PR contains a fix for that.

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.

[vbuberen]

Thanks for your contribution. Good catch.