Update announcement-banner.mdx to fix CSS letter casing (#137)

Author: stephencme

fern/products/docs/pages/navigation/announcement-banner.mdx

@@ -10,4 +10,4 @@ announcement:
     message: "🚀 New feature: Announcements are available! (<a href=\"https://buildwithfern.com/learn/docs/building-your-docs/announcements\" target=\"_blank\">Learn more</a>) 🚀"
 ```
 
-Markdown and HTML is supported in the announcement message. You can include links, images, and other formatting. [Custom css](/learn/docs/building-your-docs/custom-css-global-js#custom-css) can be used to customize the style of the announcement.
+Markdown and HTML is supported in the announcement message. You can include links, images, and other formatting. [Custom CSS](/learn/docs/building-your-docs/custom-css-global-js#custom-css) can be used to customize the style of the announcement.