[url_launcher_web] fix broken url in the README.md

The plugin's README has this line

> The web implementation of [`url_launcher`][1] 

that points to https://github.com/flutter/plugins/blob/master/packages/url_launcher/url_launcher/url_launcher which doesn't exists.

[1]: https://github.com/flutter/plugins/blob/master/packages/url_launcher/url_launcher/url_launcher 