Add missing SPDX License Identifier for mm documentation index file.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/mm/index.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/mm/index.rst b/Documentation/mm/index.rst
index 5a94a921ea40..c4e9fbacaf38 100644
--- a/Documentation/mm/index.rst
+++ b/Documentation/mm/index.rst
@@ -1,3 +1,5 @@
+.. SPDX-License-Identifier: GPL-2.0
+
 ===============================
 Memory Management Documentation
 ===============================
-- 
2.25.1