Update preload.js

Hey! I noticed you have: this.width/2 && this.height/2 instead of:
this.game.width/2 && this.game.height/2!

Don;t know if you did that on purpose for some reason, (still a beginner), but adding game in there makes it center no matter what options you put in the original generator file!

In your tutorials the loader shows up in the top right instead of center

Anyways love the generator! Can not thank you enough for it since I come from a node, yo, grunt background.
Merge pull request #3 from gbachik/patch-1

Update preload.js