removed extra 'css' code block language alias

docs/design/admonitions/index.mdx

@@ -213,7 +213,7 @@ CSS class. Update the static RGB value of `.alert--secondary` to our new variabl
 Now, let's add a color for when the page is in dark mode. The procedure is very similar to the
 previous steps, excpt we are adding a new variable to the dark mode config!
 
-```css css title="src/css/custom.css"
+```css title="src/css/custom.css"
 [data-theme='light'] {
   --note-bg: rgb(210, 216, 255);
 }