[docs] Update Terraform path in Nix docs

Summary: Fixes [[ https://linear.app/comm/issue/ENG-4455/update-nix-services-setupmd | ENG-4455 ]]. Missed this in D8567.

Test Plan: None

Reviewers: patryk, ashoat, jon

Reviewed By: patryk

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8613

docs/nix_services_setup.md

@@ -15,7 +15,7 @@ comm-dev services start
 Make sure your LocalStack resources are up to date:
 
 ```
-cd services/terraform
+cd services/terraform/dev
 ./run.sh
 ```