playground: remove ref to nacl image in README
playground: remove ref to nacl image in README

Change-Id: If35f492f6b65e16ffdd5284b7bd53ec2a033a2bd

gophergala2016/playground-plus,neollie/playground,adams-sarah/playground,adams-sarah/playground,phoorichet/playground,heshed/playground,phoorichet/playground,gophergala2016/playground-plus,golang/playground,golang/playground,neollie/playground,golang/playground,adams-sarah/playground,heshed/playground,heshed/playground,phoorichet/playground,neollie/playground,gophergala2016/playground-plus