gpiomon: capitalize 'GPIO' in error message
gpiomon: capitalize 'GPIO' in error message

Stay consistent with other error strings.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod