documentation: Format button_dev as a pointer.

The docs on creating an input device driver have an example in which
button_dev is a pointer to an input_dev struct. However, in two code
snippets below, button_dev is used as if it is not a pointer. Make these
occurrences of button_dev reflect that it is a pointer.

Signed-off-by: Nelson Penn <nelsonapenn@protonmail.com>
Link: https://lore.kernel.org/r/20220522194953.12097-1-nelsonapenn@protonmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>