Update README.md

Co-authored-by: Philippe Ombredanne <pombredanne@aboutcode.org>

Author: mjherzog

README.md

@@ -160,7 +160,8 @@ at:
 Copyright (c) the purl authors
 
 The license for **purl-spec** software is MIT: 
-- SPDX-License-Identifier: MIT-License.
+
+SPDX-License-Identifier: MIT
 - See the *LICENSE* file in this repository for the license text.
 
 The license for the ECMA-427 standard is the _ECMA Copyright License and