Update README example

[Swarthe]

Update the README example to use OpenGL 3 instead of OpenGL 2, following the suggested package flags.

Not only does this modernise the example to use a more recent OpenGL version, it also prevents possible confusion for a beginner where the example doesn't work. This is because the code uses OpenGL 2 whereas the README suggests using the opengl3 flag (see #95).

Some other minor improvements:

• Remove the use of unnecessary extensions like BlockArguments that some people might not be familiar with
• Specify some imports more precisely
• Simplify some control flow