deploy: tag the correct image for playground-goprev

Yet another location in the deploy script that was missed. Mea culpa,
haste makes waste...

Change-Id: I3b66049ac103b15c464d93020c9ed3ab909bb6b7
Reviewed-on: https://go-review.googlesource.com/c/playground/+/368194
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

Author: findleyr

deploy/deploy_goprev.json

@@ -16,7 +16,7 @@
       "entrypoint": "bash",
       "args": [
         "-c",
-        "docker build --build-arg GO_VERSION=`cat /workspace/goversion` -t gcr.io/$PROJECT_ID/playground ."
+        "docker build --build-arg GO_VERSION=`cat /workspace/goversion` -t gcr.io/$PROJECT_ID/playground-goprev ."
       ]
     },
     {