PBR example: fix normal map flag in `flags`
afaict this example is the only documentation for the flags for the PBR/standard shader. Reading the source, I can only find `FLAG_normalMap` and no `FLAG_normalTexture` (though the uniform is called `lovrNormalTexture`), so it'd be great to update the documentation to match :) (assuming that I understood the source correct and this is a valid fix)