posts/gentoo-config-luks2-grub-systemd: Reformat comment in Markdown

A common suggestion for how to add comments in Markdown is to use the
HTML comment syntax, but Hugo treats it as raw HTML and may emit a
warning.  To eliminate the warning, switch to a hacky way to add
Markdown comments at <https://www.markdownguide.org/hacks/#comments>,
which works and is simple.

content/en/posts/2022-08-21-gentoo-config-luks2-grub-systemd.md

@@ -5,11 +5,9 @@ _build:
   render: 'link'
 ---
 
-<!--
-This post is just for a redirection page to the collection with the same name.
+[This post is just for a redirection page to the collection with the same name.
 The redirection is set up in the front matter of file
-content/**/collections/gentoo-config-luks2-grub-systemd.md.
--->
+content/**/collections/gentoo-config-luks2-grub-systemd.md.]: #
 
 {{< summary.inline >}}
 {{ (.Page.GetPage "/collections/gentoo-config-luks2-grub-systemd").Summary }}