Use plan in terraform apply in readme.

Signed-off-by: Genevieve LEsperance <[email protected]>

README.md

@@ -109,7 +109,7 @@ Note: please make sure you have created the `terraform.tfvars` file above as men
 ```bash
 terraform init
 terraform plan -out=plan
-terraform apply
+terraform apply plan
 ```
 
 ### Tearing down environment