Update publish migration and config conmands in README.md

[Benjamin Fox]

The command php artisan vendor:publish --provider="Spatie\Tags\TagsServiceProvider" --tag="migrations" gave me an unable to locate publishable resources. On https://spatie.be/docs/laravel-tags/v4/installation-and-setup was able to find updated command that works. However both on README.md and on the docs site the command 'php artisan vendor:publish --provider="Spatie\Tags\TagsServiceProvider" --tag="config"' failed to locate publishable resources. But the cammand php artisan vendor:publish --provider="Spatie\Tags\TagsServiceProvider" --tag="tags-config" worked. Not much of a bug more of an update. Didn't know where to put it. Thanks for your awesome packages

[Freek Van der Herten]

I've updated this in our docs. Thanks!