Use more specific product name variable

Author: lecoursen

content/github/working-with-github-pages/troubleshooting-custom-domains-and-github-pages.md

@@ -24,7 +24,7 @@ Then, make sure the _CNAME_ file is formatted correctly.
 - The _CNAME_ filename must be all uppercase.
 - The _CNAME_ file can contain only one domain. To point multiple domains to your site, you must set up a redirect through your DNS provider.
 - The _CNAME_ file must contain the domain name only. For example, `www.example.com`, `blog.example.com`, or `example.com`.
-- The domain name must be unique across {% data variables.product.product_name %}. For example, if another repository's _CNAME_ file contains `example.com`, you cannot use `example.com` in the _CNAME_ file for your repository.
+- The domain name must be unique across all {% data variables.product.prodname_pages %} sites. For example, if another repository's _CNAME_ file contains `example.com`, you cannot use `example.com` in the _CNAME_ file for your repository.
 
 ### DNS misconfiguration