doc: some fixes in child_process.md

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, child_process

Description of change

1. assert.equal => assert.strictEqual.
2. let => const for the variable that is not reassigned.
3. Fix indentation and internal spaces in code bits.
4. Stringify buffers in code examples to prevent erroneous raw buffer outputs.
5. Fix a typo.