Add missing SPDX License Identifier for crypto documentation index file.

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

diff --git a/Documentation/crypto/index.rst b/Documentation/crypto/index.rst
index da5d5ad2bdf3..95b0870e09b8 100644
--- a/Documentation/crypto/index.rst
+++ b/Documentation/crypto/index.rst
@@ -1,3 +1,5 @@
+.. SPDX-License-Identifier: GPL-2.0
+
 ==========
 Crypto API
 ==========
-- 
2.25.1