docs: w1: Fix SPDX-License-Identifier syntax

ReST directives are introduced with two dots.
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/w1/index.rst

-. SPDX-License-Identifier: GPL-2.0
+.. SPDX-License-Identifier: GPL-2.0
 
 ================
 1-Wire Subsystem