doc: fix grammar and punctuation in cluster.md

[Rawal27]

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

[aduh95]

Please stop opening PRs faster they can be reviewed. If you do not stop, you will get blocked from the organisation.

[Rawal27]

@aduh95 Can you clarify this? I mean, these are genuine doc-level changes that I have proposed. Also, a contributor can contribute to as many as 30 pull requests at a time. Closing them without reason seems unfair to me.