Deep-Dive-Terraform:Readme update

[poesteve]

According to Pluralsight, the following course has been retired:
https://app.pluralsight.com/library/courses/terraform-getting-started

Your readme doc refers to this course being needed for the preparation of the Terraform certificate.

[Ned Bellavance]

Thanks for catching that! The link has been updated for the newer version of the Deep Dive course.