Fix broken domain authentication link
Fix broken domain authentication link

Backlog item #46. Fix broken link at bottom of page 'How to set up domain authentication'. Line 23.  Corrected link: https://sendgrid.com/docs/ui/account-and-settings/how-to-set-up-domain-authentication/
ksigler7/docs,Whatthefoxsays/docs,sendgrid/docs,Whatthefoxsays/docs,ksigler7/docs,sendgrid/docs,ksigler7/docs,sendgrid/docs,Whatthefoxsays/docs