Fix src/plan.rs license header

src/plan.rs

@@ -1,14 +1,4 @@
-// Miniscript
-// Written in 2022 by rust-miniscript developers
-//
-// To the extent possible under law, the author(s) have dedicated all
-// copyright and related and neighboring rights to this software to
-// the public domain worldwide. This software is distributed without
-// any warranty.
-//
-// You should have received a copy of the CC0 Public Domain Dedication
-// along with this software.
-// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
+// SPDX-License-Identifier: CC0-1.0
 //! A spending plan or *plan* for short is a representation of a particular spending path on a
 //! descriptor. This allows us to analayze a choice of spending path without producing any
 //! signatures or other witness data for it.