Fixed run the spider command in scrapy overview doc

Currently in http://doc.scrapy.org/en/latest/intro/overview.html there is this command to run the spider from the code snippets:

scrapy crawl mininova -o scraped_data.json -t json

But when you run it there would be an error of unknown command crawl. I think crawl is only available inside a project. For those new to scrapy normally they would just copy paste code snippets and run the command. It will be nice if this runs smoothly the first time they tried scrapy.