In downloader_middleware docs, remove reference to process_request returning a Request object

Based on experimentation, it does not seem like the download middleware process_request() method can actually return a Request object with any effect. Furthermore, the documentation below the changed lines outlines the effect of raising an IgnoreRequest exception.

With that in mind, this is a very minor change that simply updates the docs so that it is not stated that process_request() can return a Request object.