docs: gpio: intro: Improve HTML formatting

Currently the HTML output for Documentation/driver-api/gpio/intro.rst
doesn't look right. The lines that start with LOW or HIGH are formatted
in bold, while the next line after each is not bold.

With this patch, the HTML looks better.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>