deploy: tag the correct image for playground-goprev
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 <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/playground,golang/playground,golang/playground