Add nice license headers to files in examples/ and fuzzing/

Related to #2730

fuzzing/fuzz_textflow.cpp

@@ -1,4 +1,10 @@
-//License: Boost 1.0
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
 //By Paul Dreik 2020
 
 #include <catch2/internal/catch_textflow.hpp>