idr: Change documentation license

This documentation was inadvertently released under the CC-BY-SA-4.0
license.  It was intended to be released under GPL-2.0 or later.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

Documentation/core-api/idr.rst

-.. SPDX-License-Identifier: CC-BY-SA-4.0
+.. SPDX-License-Identifier: GPL-2.0+
 
 =============
 ID Allocation