Fix prod-promote typo

Why these changes are being introduced:
When attempting to merge this to the `main` branch, the `terraform plan`
failed because of a typo in the prod-promote.tpl file.

How this addresses that need:
* Change the ${ecr} reference in prod-promote.tpl to ${ecr_prod}

Side effects of this change:
None.

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/IN-568

Author: cabutlermit

files/prod-promote.tpl

@@ -1,5 +1,5 @@
-### This is the Terraform-generated prod-promote.yml workflow for the ${ecr} app repository. ###
-### If this is a Lambda repo, uncomment the FUNCTION line at the end of the document         ###
+### This is the Terraform-generated prod-promote.yml workflow for the ${ecr_prod} repository. ###
+### If this is a Lambda repo, uncomment the FUNCTION line at the end of the document.         ###
 name: Prod Container Promote
 on:
   workflow_dispatch: