deploy: fix incorrect argument in deploy_goprev.json

s/--build_arg/build-arg.

Updates golang/go#69238

Change-Id: Idd502f2f01049e2ae65949e6a1ac894cc5feaa2a
Reviewed-on: https://go-review.googlesource.com/c/playground/+/612455
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/deploy/deploy_goprev.json b/deploy/deploy_goprev.json
index 8edfbd9..3ba1ec3 100644
--- a/deploy/deploy_goprev.json
+++ b/deploy/deploy_goprev.json
@@ -21,7 +21,7 @@
       "entrypoint": "sh",
       "args": [
         "-c",
-        "docker build --build-arg GO_VERSION=`cat /workspace/goversion` --build_arg GO_BOOTSTRAP_VERSION=`cat /workspace/gobootstrapversion` -t gcr.io/$PROJECT_ID/playground-goprev ."
+        "docker build --build-arg GO_VERSION=`cat /workspace/goversion` --build-arg GO_BOOTSTRAP_VERSION=`cat /workspace/gobootstrapversion` -t gcr.io/$PROJECT_ID/playground-goprev ."
       ]
     },
     {