Add missing p tag in aside shortcode definition (#18)

This fixes an issue where an `<aside>` element with a single line would
look really weird and boxed in, compared to an `<aside>` element with
multiple lines.

This has been bugging me for a while and I finally looked up how
Benjamin Hollon handles this on his blog. Here's a good example:

https://benjaminhollon.com/musings/wireless-is-a-lie/

. When compared with my own blog, I noticed the additional `<p>` tag in
Ben's code and everything became clear. If we have multiple lines, Hugo
will automatically introduce paragraphs and the formatting will be
correct. For a single line however, Hugo would assume that it was part
of the content surrounding it. That's why the surrounding box of an
`<aside>` element with a single line would look squashed.

layouts/shortcodes/aside.html

@@ -1 +1 @@
-<aside>{{ .Inner | markdownify }}</aside>
+<aside><p>{{ .Inner | markdownify }}</p></aside>