Switch sample accounts to placeholders

To make it more obvious to users that they have to enter their own key and domain:

* Switched the sample API key to YOUR_API_KEY
* Switched the sample domain to YOUR_DOMAIN_NAME
* Switched the sample email to YOU@YOUR_DOMAIN_NAME
* Switched the sample list to LIST@YOUR_DOMAIN_NAME

Does this make sense? What did I miss?

cc @klizhentas