Quick tour broomsweep

Some typos, code highlighting, and miscellaneous formatting.

See [the end result](https://github.com/Goutte/objectory/blob/quick_tour_broomsweep/doc/quick_tour.md).

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_I like objectory._

As feedback after some setup pain, maybe the whole `objectory`-as-a-global-variable could be highlighted a bit. The `export` directive comes in handy.