docs: Fix minor spelling and grammar issues

Pre-Submission Checklist

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

[x] I assert this contribution was authored by me.
[x] I license this contribution under the license of this project.
[x] 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.
[x] This is an isolated change. No multiple changes and no unrelated changes are included.
[x] Fixed all cargo clippy warnings.
[x] Applied rustfmt.