Prevent horizontal overflow caused by "languageless" code blocks

[ironnysh]

Hi @matuzo, I read the HTMHell Advent Calendar 2023 on my mobile and noticed that code samples that aren't marked with a language are not covered by Prism's styles.

When the line is long (see here), it causes a horizontal overflow, so I thought I'd add this little CSS fix.

Thanks for the awesome content, and happy holidays!