sync: use map[any]any instead of map[interface{}]interface{} in the godoc

sync: use map[any]any instead of map[interface{}]interface{} in the godoc

It's shorter and clearer.

Change-Id: Ibfbe5a7f8d70fa884691ba9858a2c9b2122d9c50
Reviewed-on: https://go-review.googlesource.com/c/go/+/531475
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>