Docs for configuring Splash Fetcher should be base_url not base_splash_url

[rnice01]

Docs for Basic Concepts and configuring Splash fetcher state the configuration should be
fetcher: {Crawly.Fetchers.Splash, [base_splash_url: "http://localhost:8050/render.html"]}

However, the fetch method is looking for the base_url keyword in the config. As seen here

The message for the error, here, should be updated too.

[Ziinc]

It seems like there a lot of documentation errors. I'll do up a PR to consolidate efforts from #85 as well.

[oltarasenko]

This is done now!