Minor README improvements

[nexdrew]

I was reading over the docs recently and I noticed a couple problems, so I thought I'd attempt a bit of cleanup. Namely:

• Fix a few spelling and grammatical mistakes (e.g. "retrived" -> "retrieved", "then" -> "than", etc)
• Attempt to make content more consistent
• Update examples with actual output from yargs v3.15.0

Hope this helps. 😉

[bcoe]

@nexdrew thank you so much \o/

We could use a ton of help with the README.md, thanks for taking a first stab at the problem: see #92

[nexdrew]

Thanks, Ben, I'll take a look. Overall, I think the documentation is pretty good, but I may have a couple ideas about how to better organize its content, roughly following the high-level outline you mention in #92. I'll move any further discussion there.