[url_launcher] Replaced reference to shared_preferences with correct reference to url_launcher in README.

[mvanbeusekom]

The current version of the README files for the Linux, macOS, Web and Windows platform implementations contain a copy/ paste error which reference to installing the shared_preferences plugin as quoted below:

This package is endorsed, which means you can simply use shared_preferences normally. This package will be automatically included in your app when you do.

This PR fixes the mistake in the README for the platform implementations and points users to install the url_launcher plugin.

Fixes flutter/flutter#87449

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
  • This PR only contains a README documentation fix.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[BeMacized]

LGTM!

[stuartmorgan-g]

Thanks; I did a sweep for copy-paste errors, but it looks like I only caught the URLs.