Update .github/ISSUE_TEMPLATE/documentation-issue.md

Author: wiwei

.github/ISSUE_TEMPLATE/documentation-issue.md

@@ -16,5 +16,5 @@ What's incorrect? What's missing?
 **Existing Doc Link**
 If this is about something in an existing document, please provide link 
 
-**Additional context**
+## Additional context
 Add any other context about the problem here.