Update README.md

fixed spacing for Git instructions
Update CCDevice-ios.mm

On iOS, Device::setKeepScreenOn(bool value) was ignoring the value and simply always disabling the timer.
Merge pull request #8095 from SteveKanter/patch-1

Update CCDevice-ios.mm
Merge pull request #8094 from slackmoehrle/patch-2

Update README.md
Merge branch 'v3' of https://github.com/cocos2d/cocos2d-x into v3