doc: Fix a few small issues

[hakoerber]

Hey, while reading through the (awesome) stdlib docs, I found a few minor typos.

• A few typos around generic types (; vs ,)
• Use inline code formatting for code fragments
• One instance of wrong wording

[rustbot]

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

Please see the contribution instructions for more information.

[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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[Dylan-DPC]

Thanks
@bors r+ rollup

[bors]

📌 Commit 9671dd2 has been approved by Dylan-DPC

It is now in the queue for this repository.