Update README.md

gofmt'ed the example code (removes leading tabs).
Made it more similar to the example glfw3 at http://www.glfw.org/documentation.html.

The removed leading tabs make the content changes harder to see, so they are:
* Removed `defer window.Destroy()`
* Added `window.SwapBuffers()`

See if you like these changes and feel free to merge if so. [Here](https://github.com/shurcooL/glfw3-go/blob/patch-1/README.md#glfw-30-bindings-for-go)'s how it looks like.