Update Azure AD instructions to follow heading changes in Azure Portal.

> In the legacy experience, an app had Keys page. In the new experience, it has been renamed to Certificates & secrets. In addition, Public keys are referred to as Certificates and Passwords are referred to as Client secrets.

Source: https://docs.microsoft.com/en-us/azure/active-directory/develop/app-registrations-training-guide#keyscertificates--secrets

I verified by logging in Azure AD and walking through the steps.