fix: make description consistent with link-external-filter example code

[mcanouil]

The example for the link-external-filter config is confusing because it includes the word "custom" in the regex, which is not explained. This pull request updates the example to clarify that http://www.quarto.org/custom and https://www.quarto.org/custom are treated as internal links, while others are considered external. This resolves #10780.

Companion PR:

• fix: make description consistent with link-external-filter example code quarto-web#1332

[cscheid]

I don't know what's going on with gh and GitHub, but I can't seem to push to your PR branches and I swear I could in the past. I'm just going to close this and fix from a new branch.