Docs/hwmon/index: Add missing SPDX License Identifier

Add missing SPDX License Identifier for hwmon documentation index file.
Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/20230122213650.187710-5-sj@kernel.orgSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
 =========================
 Linux Hardware Monitoring
 =========================