gcp: small fix about current path

When following the instructions, based on previous commands, make image
will be executed on podvm-mkosi directory again.

Signed-off-by: Beraldo Leal <bleal@redhat.com>

src/cloud-api-adaptor/gcp/README.md

@@ -135,6 +135,7 @@ kubectl apply -k "github.com/confidential-containers/operator/config/samples/ccr
 Build and push docker image to a registry:
 
 ```bash
+cd ../ # make sure you are outside the podvm-mkosi directory
 export registry=quay.io/${QUAY_USER} # If you are using local registry: LOCAL_IP:PORT.
 export RELEASE_BUILD=true
 make image