Section numbers are off by one after horizontal rules

[fitzgen]

If one has this SUMMARY.md:

# Summary

- [Chapter 1](./chapter_1.md)
--------------------------------------------------------------------------------
- [Chapter 2](./chapter_2.md)
--------------------------------------------------------------------------------
- [Chapter 3](./chapter_3.md)

then one ends up with this bogus section numbering:

[fitzgen]

If someone could drop some pointers for me in here, I would be happy to try and whip up a fix :)

[mattico]

My first suggestion would be to see if this happens with upstream pulldown-cmark. I can't think of anywhere in mdbook itself that is likely to cause this at the moment, but hopefully I can be more helpful if it's not a bug in pulldown-cmark.

[badboy]

The Table of Contents is not rendered by pulldown-cmark, but by the toc helper and previously parsed in summary.rs, which is where I think an off-by-one error is hidden.