Documentation fixes for composer-plugin type

As documented in https://github.com/composer/composer/blob/master/doc/articles/custom-installers.md#composerjson:

> the type attribute must be composer-plugin.

In addition use getPackageBasePath instead of getInstallPath, as LibraryInstaller uses getPackageBasePath for uninstall: https://github.com/composer/composer/blob/master/src/Composer/Installer/LibraryInstaller.php#L126