Update code example to match new sdl2.nim syntax (#13924)
Update code example to match new sdl2.nim syntax (#13924)

* Update code example to match new sdl2.nim syntax

Signed-off-by: Hieu Luu Danh <f43aafa5ac7cc9fcf346e2835b515cee631156b3@vivu.asia>

* Modify on recommendation of @alaviss

- Removed trailing whitespaces
- Detailed how to compile libnimhcr and libnimrtl
- Fixed some logic in example code

* Modify following recommendations of @alaviss

- Rewording so that it conforms to Windows/Linux/MacOS

Signed-off-by: Hieu Luu Danh <f43aafa5ac7cc9fcf346e2835b515cee631156b3@vivu.asia>
dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim