Update README.md - max_comments is total. max_parents is only comments

[pato-pan]

An example that explains that max_comments is the total including replies, and max_parents is only the comments. I only found a clarification of what this means on the discord and a Reddit post.

Description of your pull request and other information

Adds one more example to the youtube:max_comments extractor args. Hopefully it is concise enough

Template

Before submitting a pull request make sure you have:

• [x] At least skimmed through contributing guidelines including yt-dlp coding conventions
• [x] Searched the bugtracker for similar pull requests

In order to be accepted and merged into yt-dlp each piece of code must be in public domain or released under Unlicense. Check those that apply and remove the others:

• [x] I am the original author of the code in this PR, and I am willing to release it under Unlicense
• [ ] I am not the original author of the code in this PR, but it is in the public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request? Check those that apply and remove the others:

• [x] Updating documentation