Fix the issue of extra </span> after the image

It’s a know issue reported at
https://github.com/robwierzbowski/jekyll-picture-tag/issues/23

It is caused by the unwant `<p>` tag around the `<span>` tag.

As the markdown processing will wrap the `<span>` element with a `<p>` tag,
so use the {::nomarkdown} {:/} block to skip the markdown processing to
avoid the extra `<p>` tag.