Add new required(ish) "parser" parameter to markdownlint custom rule examples in Content Linter README.md.

[DavidAnson]

Why:

parser is a new required (tho it's okay if it's not present for now) property on markdownlint custom rules. This PR adds it to the two relevant examples.

Closes:

N/A

What's being changed (if available, include any code snippets, screenshots, or gifs):

README

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[welcome]

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[nguyenalex836]

@DavidAnson Thanks so much for opening a PR! I'll get this triaged for review ✨

[gracepark]

Hi @DavidAnson thanks for reaching out and for the contribution! I did notice it recently when I was looking at updating one of our custom rules. I'll make sure to update them all and get this added. Thanks again!

[github-actions]

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[DavidAnson]

FYI @gracepark if you are a rule maintainer, this new property does nothing at the moment, but I intend to use it soon to avoid loading the markdown-it parser once I have migrated all the core rules to micromark. Of course, if there are rules that need markdown-it tokens, it will be used and the tokens will still be available. From what I saw, GitHub rules take advantage of it, so there may not be a performance benefit for you folks. micromark is not yet available for public use, but that is also planned. Finally, note that if a rule doesn't use tokens at all, it can declare that.

[gracepark]

@DavidAnson - yes I'm apart of the team that maintains the rules. I'll let the others know as well. Thanks for the heads up.