fix: make Novel543Parser toc URL construction idempotent

[kuwoyuki]

First path segment on those websites is always the book ID, so just take the first segment and append /dir to it.
This avoids confusing errors if you open WebToEpub on the TOC or chapter page

[gamebeaker]

@kuwoyuki do you want to add yourself to the "contributors" section of packages.json and updated the "Credits" section of readme.md with your name?

[kuwoyuki]

@kuwoyuki do you want to add yourself to the "contributors" section of packages.json and updated the "Credits" section of readme.md with your name?

done

[dteviot]

Thank you.