Remove weird Cell example from InvariantType docs

[leejunseok]

Should fix #20147

P.S. This is my first PR in the history of ever. Please let me know if I'm doing anything wrong.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @huonw (or someone else) soon.

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[huonw]

Hi there! Thanks for the PR. Everything looks in order, but I'm not sure this example deserves to be removed.

cc @steveklabnik

[leejunseok]

Woops, I'm sorry, I probably should've asked around a little before making the changes.

[steveklabnik]

I don't know much about InvariantType, but I think this looks good to me 👍

[steveklabnik]

@leejunseok some odd commits have crept in here, can you rebase and sqaush? r=me after

[leejunseok]

hrm, sorry git still isn't quite natural to me, I'm trying to get my history sorted out.

[leejunseok]

I didn't know how to sync rust-lang/rust and my own rust (along with my local repo) and I ended up doing something very strange to my repo. I've figured it out now, but my history was too mangled for me to save. I'll open another PR!

Sorry about the inconvenience!