Fix header

Co-Authored-By: tmandry <[email protected]>

Author: mark-i-m

src/traits/index.md

@@ -43,7 +43,7 @@ Trait solving in rustc is based around a few key ideas:
 
 > This is not a complete list of topics. See the sidebar for more.
 
-### Ongoing work
+## Ongoing work
 The design of the new-style trait solving currently happens in two places:
 
 **chalk**. The [chalk][chalk] repository is where we experiment with new ideas