Fix instructions on how to deploy from master. (#966)

After 4516141,
make docker-push-release does not push the "latest" tag anymore.
Following the instructions have the effect of pushing only the VERSION tag but not latest.
make docker-push pushes latest, having the desired effect.

Signed-off-by: Federico Paolinelli <fpaoline@redhat.com>

README.md

@@ -104,7 +104,7 @@ Currently the most reliable way of installing Gatekeeper is to build and install
    * Build and push your Docker image:
       ```sh
       make docker-buildx REPOSITORY="$DESTINATION_GATEKEEPER_DOCKER_IMAGE"
-      make docker-push-release REPOSITORY="$DESTINATION_GATEKEEPER_DOCKER_IMAGE"
+      make docker-push REPOSITORY="$DESTINATION_GATEKEEPER_DOCKER_IMAGE"
       ```
    * Finally, deploy:
      ```sh