Docs: Fix format of headings in String::reserve

[dbrgn]

It was inconsistent with other doc comments in the same module (and the rest of the rust std docs).

[rustbot]

r? @cuviper

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[rustbot]

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 @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+ rollup

[bors]

📌 Commit 4f36673 has been approved by cuviper

It is now in the queue for this repository.