fix a comment in UnsafeCell::new

[RalfJung]

There are several safe methods that access the inner value: into_inner has existed since forever and get_mut also exists since recently. So this comment seems just wrong. But &self methods return raw pointers and thus require unsafe code (though the methods themselves are still safe).

[rustbot]

r? @joshtriplett

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[rustbot]

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[JohnTitor]

Sounds sensible!
@bors r+ rollup

[bors]

📌 Commit dad3270 has been approved by JohnTitor

It is now in the queue for this repository.