Fix the generator example for pin!()

[mbartlett21]

The previous generator example is not actually self-referential, since the reference is created after the yield.

CC #93178 (tracking issue)

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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[rust-highfive]

r? @joshtriplett

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[Dylan-DPC]

@bors r+ rollup

[bors]

📌 Commit 6d523e9 has been approved by Dylan-DPC

[JohnTitor]

Good catch, thanks!

(EDIT: sorry, I was opening a tab while understanding the context here then submitted a review. I was too late here!)