Improve JS3 Week1 lesson on HTTP headers and status codes

[shieldo]

What does this change?

Module: JavaScript Core 3 Week(s): 2

Checklist

• [x] I have read the contributing guidelines
• [x] I have checked my spelling and grammar with an automated tool
• [x] I have previewed my changes to check the markdown renders as I intend
• [x] I have run my code to check it works
• [x] My changes follow our Style Guide

Description

Improved and tidied the sections on headers and status codes, correcting some factual inaccuracies (such as what constitutes a "header"), giving examples, and hopefully making things generally clearer while keeping things at the appropriate level of detail.

Who needs to know about this?

Rendered Pages