Correct docs for `center` in Emitter#at
Correct docs for `center` in Emitter#at

Target is either

- obj.center
- obj.world + anchor * size / 2

photonstorm/phaser-ce,yupaul/phaser-ce,photonstorm/phaser-ce,MasahiroWard/MVPgamedev,yupaul/phaser-ce,natlot/phaser-ce,photonstorm/phaser-ce,MasahiroWard/MVPgamedev,natlot/phaser-ce,MasahiroWard/MVPgamedev,yupaul/phaser-ce,natlot/phaser-ce,MasahiroWard/MVPgamedev,natlot/phaser-ce,photonstorm/phaser-ce,yupaul/phaser-ce