stb_image: add trailing semicolon in usage example

[lunasorcery]

Tiny little one-char change to stop the basic usage example from throwing a missing-semicolon error. This one's bugged me for years, I always end up metaphorically stubbing my toe on it, each time I copy-paste the usage example into my own code.

Also wasn't sure whereabouts in the credits to put my name, or if such a tiny change even warrants it, let me know :)