We need a single empty line between the code-block state and the text
in the block, otherwise the rST is invalid and the entire block will be
dropped, as is currently the case on the website.

While we're at it, remove some needless colons from these code-blocks as
well. They're not needed, and we usually don't have these in the docs.

Fixes: a2a8c6a3

 ("docs: Add some documentation of game GL buffer object mapping behavior.")
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <!9710>