fixing GL namespace in readme

@nilium

Fixing the namespace of `Gl` to `GL` which the `opengl-core` gem includes. 

I was getting this error:
```
lib/test.rb:25:in `block in <main>': uninitialized constant Gl (NameError)
	from lib/test.rb:22:in `loop'
	from lib/test.rb:22:in `<main>'
```

Also updated the blocks to do/end syntax since they are multiline. Thanks!