Sorry, there are no formatting options for the Table of Contents.
This is exactly the type of thing that we want authors to not worry about or spend any time on. So, we're opinionated here.
I know exactly how you feel -- I've written technical books too, and if there was a workaround, I would have used it too, and I would have noticed, just as you have. However, it provides essentially no value to your readers, and it is a slippery slope: from this, it leads to mucking around with image positioning, widows and orphans, margins, fonts, etc, etc, and it's a great way to procrastinate instead of writing :)
Now, that said, there are some very big hammer workarounds:
- Change the page size here (the most options are in the Custom style): https://leanpub.com/YOURBOOK/edit_formatting
- When you are 100% done the book, export InDesign and have a designer make a beautiful book: https://leanpub.com/YOURBOOK/in_design
Neither of these are things I'd recommend while a book is in-progress, of course :)