test,doc: adjust async-hooks coverage doc for lint

First header should be a first-level header according to our lint rules.
Make it so in prepartion for applying the markdown linting to the test
directory.

PR-URL: #22221
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Jon Moss <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: Khaidi Chu <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: George Adams <[email protected]>

test/async-hooks/coverage.md

@@ -1,4 +1,4 @@
-## AsyncHooks Coverage Overview
+# AsyncHooks Coverage Overview
 
 Showing which kind of async resource is covered by which test: