Add missing comma (mmistakes#3318)

Co-authored-by: Yuchen Zhong <yuchen21@fb.com>

docs/_docs/05-configuration.md

@@ -28,7 +28,7 @@ theme: minimal-mistakes-jekyll
 Easily change the color scheme of the theme using one of the provided "skins":
 
 ```yaml
-minimal_mistakes_skin: "default" # "air", "aqua", "contrast", "dark", "dirt", "neon", "mint", "plum" "sunrise"
+minimal_mistakes_skin: "default" # "air", "aqua", "contrast", "dark", "dirt", "neon", "mint", "plum", "sunrise"
 ```
 
 **Note:** If you have made edits to the theme's CSS files be sure to update [`/assets/css/main.scss`](https://github.com/mmistakes/minimal-mistakes/blob/master/assets/css/main.scss) to include `@import "minimal-mistakes/skins/{{ site.minimal_mistakes_skin | default: 'default' }}"; // skin` before the `minimal-mistakes` import.