Spelling and styling of `doc/policy/maintenance.md`

What does this MR do?

Docs cleanup. I read through the document to upgrade my local install, and came across an extra .. I then ran the spell-checking tool Grammarly on the document and fixed a few other findings.

Screenshots

N/A

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

N/A

Security

N/A