docs: Update install.md (#3191)

Signed-off-by: Asya-kawai <33394138+Asya-kawai@users.noreply.github.com>

website/versioned_docs/version-v3.14.x/install.md

@@ -56,7 +56,7 @@ Currently the most reliable way of installing Gatekeeper is to build and install
    * Build and push Gatekeeper image:
       ```sh
       export DESTINATION_GATEKEEPER_IMAGE=<add registry like "myregistry.docker.io/gatekeeper">
-      make docker-buildx REPOSITORY=$DESTINATION_GATEKEEPER_DOCKER_IMAGE OUTPUT_TYPE=type=registry
+      make docker-buildx REPOSITORY=$DESTINATION_GATEKEEPER_IMAGE OUTPUT_TYPE=type=registry
       ```
 
       > If you want to use a local image, don't set OUTPUT_TYPE and it will default to `OUTPUT_TYPE=type=docker`.