doc: fix grammar and punctuation in cluster.md - #65238

Description

Fix minor grammar and punctuation issues in the cluster module documentation.

Changes:

• Correcting singular/plural usage (method → methods).
• Adding missing context and punctuation around references to the primary process and worker.
• Adding a missing comma in an introductory phrase.
• Correcting the grammar in the description of cluster.setupPrimary() defaults.

These changes are documentation-only and do not affect Node.js functionality or behavior.

Checklist

• Documentation-only changes
• No functional changes
• No tests required