title: YAML header of markdown file generates odd table on top of document
body: <p>I get a weird output from my YAML header which I do not intend. 
  Instead I expect the normal output with title, author and the table of contents of course. </p>
  
  <p>The following code produces the following output:</p>
  
  <pre><code>---
  title: General Stuff
  author: Me
  output:
      html_document:
      toc: true # table of content true
      toc_depth: 3  # upto three depths of headings (specified by #, ## and ###)
      number_sections: true  ## if you want number sections at each table header
      theme: united
  ---
  </code></pre>
  
  <p>generates this output:</p>
  
  <p><a href="https://i.imgur.com/N9HVbxf.png" rel="nofollow noreferrer"><img src="https://i.imgur.com/N9HVbxf.png" alt=""></a></p>
  
  <p>I got this output from GitHub where the file is displayed. Maybe the issue is that GitHub uses some interesting markdown version...?</p>