Update cname-domain-verification.mdx

Adding a reference to _domainkey as this is the most common issue

Author: ngayerie

src/content/docs/dns/manage-dns-records/troubleshooting/cname-domain-verification.mdx

@@ -10,6 +10,7 @@ head:
 ---
 
 When configuring services from external providers - such as email services, for example - it is possible that they require you to verify your domain by placing a CNAME record at your zone.
+(for example: `<value>._domainkey.example.com` CNAME `<hostname>.<service provider domain>`)
 
 Consider the following sections if this is not working correctly for you.