Update Foundation README.md

Author: adamblanchard

courses/Foundation/README.md

@@ -2,4 +2,4 @@
 > Work in progress. Proposed content defined [here](https://docs.google.com/document/d/151MLm-8WA6jSk0-9JhBTuG1xZ9Fo9HRLplJx6Bhps6A/edit?tab=t.0).
 
 # Modules
-## [HTML & CSS](./HTML-CSS)
+### [HTML & CSS](./HTML-CSS)