composer.json rename muplugins to muplugin

I think `type:wordpress-muplugins` is a typo, because composer/installers ([source](https://github.com/composer/installers/blob/master/src/Composer/Installers/WordPressInstaller.php)) uses  `muplugin` and one other popular WP project ([roots/bedrock](https://github.com/roots/bedrock/blob/master/composer.json)) uses `muplugin` as well.

Not sure if this is a big problem but I think it would be nice to keep the naming the same as other projects so there's no confusion about which one is the correct one.