Fix return type

[riesjart]

Spatie\Crawler\Crawler::startCrawling() accepts any instance of Psr\Http\Message\UriInterface.
When e.g. an instance of Spatie\Url\Url has been provided, everything goes well until Spatie\Crawler\Handlers\CrawlRequestFulfilled::getBaseUrl() fails to return an instance of GuzzleHttp\Psr7\Uri instead of the PSR interface. The code doesn't depend on the concrete Guzzle Uri however.

[freekmurze]

Thanks!