Fix some small typos and errors in docs

I've been reading through the Scrapy docs and thought I'd clean up some typos, trailing whitespace, and small mistakes along the way.

I haven't changed anything significant, it's mostly single word/character changes, except one part in `MapCompose` documentation: the way it was written seemed to suggest that the second function receives the whole iterable as an argument, so I rewrote it (hopefully in a better way) in a separate commit.

I also didn't intend to fix the trailing whitespace initially, so I've only touched it in the files that I've already changed for a different reason.