cmd/present: fix obsolete flag in security warning

The warning printed when present exposes local code execution recommends
using -play_socket=false, but that flag no longer exists.

Recommend -use_playground=true instead, which causes snippets to run
using the Go playground rather than on the local machine.

Fixes golang/go#80569

Change-Id: Ic24a0049585cdc2c1a921d2ad96e9fdbb3ebd206
Reviewed-on: https://go-review.googlesource.com/c/tools/+/805920
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Sean Liao <sean@liao.dev>

cmd/present/main.go

diff --git a/cmd/present/main.go b/cmd/present/main.go
index 6c7a40c..36786ce 100644
--- a/cmd/present/main.go
+++ b/cmd/present/main.go
@@ -117,7 +117,7 @@
 and port will have access to this machine as the user running present.
 
 To avoid this message, listen on localhost, run with -play=false, or run with
--play_socket=false.
+-use_playground=true.
 
 If you don't understand this message, hit Control-C to terminate this process.