Fix orientation of teapot in `wgpu_instancing.rs` after update to 0.5

I just realised that I forgot to correct the orientation of the teapot in this example to account for wgpu's switch to having the y axis increase upwards (rather than downwards). Previously the teapot examples all used a view matrix with an inverted axis in order to account for the downwards y axis, as the original teapot vertices were intended for an upward y axis. This has been correct in the wgpu_teapot* examples in #594, but I forgot to correct it in wgpu_instancing.rs. This is a reminder to myself (or anyone) to fix it.