Fix license discrepancy

All files should be Apache + MIT dual. As author of this file I don't believe any further approvals are needed.

Thanks to @dominikh for flagging.

src/offset.rs

@@ -1,16 +1,5 @@
-// Copyright 2022 The Kurbo Authors
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
+// Copyright 2022 the Kurbo Authors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
 
 //! Computation of offset curves of cubic Béziers, based on a curve fitting
 //! approach.