Docs/driver-api/index: Add missing SPDX License Identifier

Add missing SPDX License Identifier for driver-api documentation index
file.
Signed-off-by: SeongJae Park <sj@kernel.org>
Link: https://lore.kernel.org/r/20230122213650.187710-3-sj@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
 ==============================
 Driver implementer's API guide
 ==============================