Doc typo/grammar fix.

[RustEnthusiast]

Just fixes a grammatical error in NonNull's unstable as_uninit_ref and as_uninit_mut methods.

[rustbot]

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

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[rustbot]

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If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. 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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[cuviper]

Thanks!

@bors r+

[bors]

📌 Commit 2f29108 has been approved by cuviper

It is now in the queue for this repository.

[cuviper]

@bors rollup