Correct code block in installation docs for Snap (#20440)

Without this, it was rendering on the site like: "sh snap install gitea", instead of: "snap install gitea"

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

docs/content/doc/installation/from-package.en-us.md

@@ -47,9 +47,9 @@ pacman -S gitea
 
 There is a [Gitea Snap](https://snapcraft.io/gitea) package which follows the latest stable version.
 
-``sh
+```sh
 snap install gitea
-``
+```
 
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