playground: remove ref to nacl image in README

Change-Id: If35f492f6b65e16ffdd5284b7bd53ec2a033a2bd
diff --git a/README.md b/README.md
index bf7f902..a9a2a22 100644
--- a/README.md
+++ b/README.md
@@ -6,9 +6,7 @@
 ## building
 
 ```
-# build the golang:nacl base image
-docker build -t golang:nacl gonacl/
-# build the sandbox server image
+# build the sandbox image
 docker build -t sandbox sandbox/
 ```