fix guidance_rescale docstring

[codinggosu]

What does this PR do?

Fixes #4864

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Who can review?

tagging: @stevhliu for the review

[codinggosu]

One question:

in StableDiffusionGLIGENPipeline the parameters to __call__ do not contain guidance_rescale, yet the docstring has string containing guidance_rescale.

Should I just remove the line regarding guidance_rescale altogether?

Currently, in this PR, I have just replaced the wrong default value for the correct one (0.7 -> 0.0)
If the reviewers think that it would be better to just remove this line for StableDiffusionGLIGENPipeline, please tell me

[patrickvonplaten]

Thanks a lot!