Fix doc "Create Custom Feature Extractor", normalized_images

[riiswa]

Description

I don't think that the current way indicated in the documentation is the right way to prevent stable-baselines3 from normalizing the images. I've updated the documentation, with a version that works (I've tested it).

If needed, I can create an issue, and link it to this PR.

Type of change

Please delete options that are not relevant.

• This change requires a documentation update

Screenshots

Please attach before and after screenshots of the change if applicable.

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[pseudo-rnd-thoughts]

@BolunDai0216 Is this correct?