docs: gpio: driver.rst: fix a bad tag

With ReST, [foo]_ means a reference to foo, causing this warning:

    Documentation/driver-api/gpio/driver.rst:419: WARNING: Unknown target name: "devm".

Fix it by using a literal for the name.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>