Fix minor bug in SD3 img2img test

[a-r-r-o-w]

What does this PR do?

vae image processor takes inputs in range [0, 1] for images. previously, we had an additional line that makes the range [0.5, 1]. while that works, i think this is the more correct thing to do.

context: https://github.com/huggingface/diffusers/pull/8709#discussion_r1661780194

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.

@yiyixuxu