Small edits in the 2nd main area of the tutorial

Andrea, these are mostly style and idiom edits again, except for the Combinators section, which felt like it was too dense and sparse for a true Factor newbie to follow. I tried to expand it a bit with some extra breakdown on what's going on, and some extra listener examples to slow down the pace a bit.

Also in that section, the example `prime?` word answers that 2 is not a prime number. We should either:

a) point it out to the user as a limitation of our toy example
b) fix it
c) point it out and invite the user to try and fix it

I think I prefer c), but I'll defer to you on this.

Cheers,
Sean