Fix name of OpenTofu variables file

OpenTofu still loads variables from `terraform.tfvars` [1].

[1] https://opentofu.org/docs/language/values/variables/#variable-definition-precedence

Author: priteau

README.md

@@ -82,7 +82,7 @@ And generate secrets for it:
 
 Create an OpenTofu variables file to define the required infrastructure, e.g.:
 
-    # environments/$ENV/tofu/tofu.tfvars:
+    # environments/$ENV/tofu/terraform.tfvars:
 
     cluster_name = "mycluster"
     cluster_networks = [