sleep_ms is deprecated since 1.6
sleep_ms is deprecated since 1.6

according to https://doc.rust-lang.org/std/thread/fn.sleep_ms.html and
the use line above uses sleep. This fixes this for first time users of the library.

rust-embedded/rust-sysfs-gpio,marjakm/rust-sysfs-gpio,nastevens/rust-sysfs-gpio