bindings: cxx: examples: use seconds for timeout in line.event_wait()

bindings: cxx: examples: use seconds for timeout in line.event_wait()

While the event_wait() method takes ::std::chrono::nanoseconds as the
timeout argument, we don't actually need to use the same type - we can
use less fine-grained duration types. Switch to using seconds in the
gpiomoncxx example to showcase this implicit conversion.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>