Fix markdown message support

[Gennadiyev]

According to ntfy.sh docs, the Markdown header should be yes instead of true.

[MatthewCane]

The docs state that:

To enable Markdown, set the X-Markdown header (or any of its aliases: Markdown, or md) to true (or 1 or yes), or set the Content-Type header to text/markdown

I don't feel the need to change this, unless you are finding that the Markdown feature isn't working as you would expect?