cmd/tip: enable playground on tip.golang.org

Updates golang/go#28536

Change-Id: I3ece72aacd4635f4580be404672394a47791a1e8
Reviewed-on: https://go-review.googlesource.com/c/146917
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/tip/godoc.go b/cmd/tip/godoc.go
index a01b8ca..4332c35 100644
--- a/cmd/tip/godoc.go
+++ b/cmd/tip/godoc.go
@@ -48,7 +48,7 @@
 	}
 
 	godocBin := filepath.Join(goPath, "bin/godoc")
-	godoc := exec.Command(godocBin, "-http="+hostport, "-index", "-index_interval=-1s")
+	godoc := exec.Command(godocBin, "-http="+hostport, "-index", "-index_interval=-1s", "-play")
 	godoc.Env = []string{"GOROOT=" + goDir}
 	// TODO(adg): log this somewhere useful
 	godoc.Stdout = os.Stdout