docs: i2c: 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/i2c/busses/index.rst

-. SPDX-License-Identifier: GPL-2.0
+.. SPDX-License-Identifier: GPL-2.0
 
 ===============
 I2C Bus Drivers

Documentation/i2c/index.rst

-. SPDX-License-Identifier: GPL-2.0
+.. SPDX-License-Identifier: GPL-2.0
 
 ===================
 I2C/SMBus Subsystem