docs: Fix minor spelling and grammar issues

[anholt]

Pre-Submission Checklist

When you submit a PR, please make sure your PR satisfies the following checklist:

• I assert this contribution was authored by me.
• I license this contribution under the license of this project.
• The PR title describes your changes briefly and uses the present tense, without a trailing full stop.
• The PR description describes the reason why we need the change.
• This is an isolated change. No multiple changes and no unrelated changes are included.
• Fixed all cargo clippy warnings.
• Applied rustfmt.

[nuta]

Great. Thanks!