[Improvement] Update https://api.octocorp.ghe.com to a more generic https://api.{subdomain}.ghe.com example (#57788)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Joe Clark <31087804+jc-clark@users.noreply.github.com>

Author: 3 people

data/reusables/enterprise-migration-tool/set-target-api-url.md

@@ -3,13 +3,18 @@
    * If you're using Terminal, use the `export` command.
 
      ```shell copy
-     export TARGET_API_URL="https://api.{% data variables.enterprise.data_residency_example_domain %}"
+     export TARGET_API_URL="https://api.SUBDOMAIN.ghe.com"
      ```
 
    * If you're using PowerShell, use the `$env` command.
 
      ```shell copy
-     $env:TARGET_API_URL="https://api.{% data variables.enterprise.data_residency_example_domain %}"
+     $env:TARGET_API_URL="https://api.SUBDOMAIN.ghe.com"
      ```
 
+   > [!IMPORTANT]
+   > Ensure to replace `SUBDOMAIN` with your enterprise's subdomain.
+   >
+   > For example, if your enterprise's subdomain is `acme`, the `TARGET_API_URL` value would be `https://api.acme.ghe.com`.
+
    You'll use this variable with the `--target-api-url` option in commands you run with the {% data variables.product.prodname_cli %}.