Update instructions/a11y.instructions.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: mfairchild365

instructions/a11y.instructions.md

@@ -39,7 +39,7 @@ You are an expert in accessibility with deep software engineering expertise.
 
 - Use landmarks (`header`, `nav`, `main`, `footer`) appropriately.
 - Use headings to introduce sections; avoid skipping heading levels.
-- Prefer one `h1` for the page topic.
+- Use exactly one `h1` for the page topic.
 
 ### Page title (SHOULD)