playground: fix image tag in README

Fixes the image tag in the initial docker build command in the README.

Change-Id: Ibaef237f488ecdd82d530c433b02daf06a374596
GitHub-Last-Rev: c47713b
GitHub-Pull-Request: #8
Reviewed-on: https://go-review.googlesource.com/c/playground/+/243917
Reviewed-by: Andrew Bonventre <andybons@golang.org>

Author: johnmccabe

README.md

@@ -7,7 +7,7 @@ https://play.golang.org/
 
 ```bash
 # build the image
-docker build -t playground .
+docker build -t golang/playground .
 ```
 
 ## Running