fix: Add use statements to doctest

uhhh that doesn't fail on my computer but. you know github. i forgive
you. here you go my friend

src/lib.rs

@@ -10,6 +10,9 @@
 //! Then, you can use something like this example in your binary...
 //!
 //! ```no_run
+//! use linux_max6675::Max6675;
+//! use std::time::Duration;
+//!
 //! let mut max = Max6675::new("/dev/spidev0.0")?;
 //!
 //! std::thread::sleep(Duration::from_secs(3));