docs: typo fix - sentence was missing a verb (#275)

[ci skip]

README.md

@@ -74,7 +74,7 @@ A few required options should be set, typically via the appsettings.json file.
  // If you don't set this in config, you will need to press "y" whenever the application starts
  "AcceptTermsOfService": true,
 
- // You must at least one domain name
+ // You must specify at least one domain name
  "DomainNames": [ "example.com", "www.example.com" ],
 
  // You must specify an email address to register with the certificate authority