Fixed typos/errors in 2D chapter

Couple of queries on this one:

Wasn't sure what you meant by:
> designed to take the game out of building simple 2D games

I guessed that 'game' should have been 'pain'?

And secondly, this paragraph:

> To turn off gravity, you can either set the global gravity values `Q.gravityX` and `Q.gravityY` or set the gravitiy multiplier on the Sprite itself (setting either `p.gravityX` or `p.gravityY` to 0 will disable the effect of those.

When using the 2d component I found that the property on the Sprite was just gravity, without specific directions - is this a future change that should be listed in errata or something that should be changed here to reflect the current engine state?

I also tried to describe the concept of a viewport a bit clearer.