SPDX (#2619)

packages/api-contract/src/base/Contract.ts

@@ -1,6 +1,5 @@
 // Copyright 2017-2020 @polkadot/api-contract authors & contributors
-// This software may be modified and distributed under the terms
-// of the Apache-2.0 license. See the LICENSE file for details.
+// SPDX-License-Identifier: Apache-2.0
 
 import { ApiTypes, DecorateMethod, ObsInnerType } from '@polkadot/api/types';
 import { AccountId, Address, ContractExecResult } from '@polkadot/types/interfaces';