Improved grammar and sentence structure in the JS programming guide.

"The API is completely compatible between Cocos2d-html5 and Cocos2d-x." is not true. The API is completely compatible between Cocos2d-html5 and Cocos2d-x JSB. Cocos2d-x only has cpp API before JS binding. It should be "The API is completely compatible between Cocos2d-html5 and Cocos2d-x JSB."