Remove line breaks in section title in TOC (LaTeX)

Hello,

If I use line breaks in the section title, then the TOC in LaTeX also contains those line breaks. This shouldn't happen. This patch removes line breaks in the section title in the TOC in LaTeX output.

Example:
```markdown
## JSON\
filters
```
produces
```latex
\subsection[JSON filters]{JSON\\filters}\label{json-filters}
```