title: Change toc_depth field in bookdown pdf output
body: <p>I am creating a book in PDF with <code>bookdown</code>, and I would like to change the number of the <code>toc_depth</code> field when converting the book to PDF. This is my <code>_output-yml</code>:</p>
  
  <pre><code>bookdown::gitbook:
    css: style.css
    config:
      toc:
        before: |
          &lt;li&gt;&lt;a href="./"&gt;A Minimal Book Example&lt;/a&gt;&lt;/li&gt;
        after: |
          &lt;li&gt;&lt;a href="https://github.com/rstudio/bookdown" target="blank"&gt;Published with bookdown&lt;/a&gt;&lt;/li&gt;
        toc_depth: 5
      download: ["pdf", "epub"]
  bookdown::pdf_book:
    includes:
      in_header: preamble.tex
    latex_engine: xelatex
    citation_package: natbib
    keep_tex: yes
  bookdown::epub_book: default
  </code></pre>
  
  <p>I added <code>toc_depth: 5</code> with no results. How can I do it?</p>