fix(legal): Workaround reuse tool

I noticed while using https://reuse.software/
this false positive error:

    reuse.extract - ERROR - Could not parse 'MIT\n','
    reuse.extract - ERROR - \
      '.../FreeRTOS-Kernel/.github/scripts/kernel_checker.py' \
      holds an SPDX expression that cannot be parsed, skipping the file

Origin: #1322
Relate-to: #1321
Signed-off-by: Philippe Coval <[email protected]>

Author: rzr

.github/scripts/kernel_checker.py

@@ -125,7 +125,7 @@
     ' * FreeRTOS Kernel <DEVELOPMENT BRANCH>\n',
     ' * Copyright (C) 2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.\n',
     ' *\n',
-    ' * SPDX-License-Identifier: MIT\n',
+    ' * SPDX', '-License-Identifier: MIT\n',
     ' *\n',
     ' * Permission is hereby granted, free of charge, to any person obtaining a copy of\n',
     ' * this software and associated documentation files (the "Software"), to deal in\n',