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>
diff --git a/Documentation/core-api/idr.rst b/Documentation/core-api/idr.rst
index d351e88..a273805 100644
--- a/Documentation/core-api/idr.rst
+++ b/Documentation/core-api/idr.rst
@@ -1,4 +1,4 @@
-.. SPDX-License-Identifier: CC-BY-SA-4.0
+.. SPDX-License-Identifier: GPL-2.0+
 
 =============
 ID Allocation