Make REUSE ignore section heading as a copyright statement (rust-lang#1659)

Author: pietroalbini

src/conventions.md

@@ -29,7 +29,10 @@ pass the <!-- date-check: nov 2022 --> `--edition=2021` argument yourself when c
 
 <a name="copyright"></a>
 
+<!-- REUSE-IgnoreStart -->
+<!-- Prevent REUSE from interpreting the heading as a copyright notice -->
 ### Copyright notice
+<!-- REUSE-IgnoreEnd -->
 
 In the past, files began with a copyright and license notice. Please **omit**
 this notice for new files licensed under the standard terms (dual