Update formats.qmd typo

css should be scss

Author: ryjohnson09

docs/extensions/formats.qmd

@@ -93,7 +93,7 @@ contributes:
 
 The custom HTML format defined here is very simple. It takes the base `html` format, turns on the table of contents by default, and sets the theme as `yeti` along with a `custom.scss` file for additional customizations:
 
-```{.css filename="_extensions/lexdoc/custom.css"}
+```{.css filename="_extensions/lexdoc/custom.scss"}
 /*-- scss:defaults --*/
 
 /* TODO: Customize appearance with SCSS variables */