Merge pull request #6 from mogenson/master

fixup serial output example so that it builds correctly

src/hello-world/02.00.UART.md

@@ -24,12 +24,13 @@ To read and write to the serial bus from your computer, you will need to configu
 ## Code
 
 ``` rust
+use core::fmt::Write;
 use microbit::hal::prelude::*;
 use microbit::hal::serial;
 use microbit::hal::serial::BAUD115200;
 // -- snip --
 if let Some(p) = microbit::Peripherals::take() {
-    let mut gpio = p.GPIO.split();
+    let gpio = p.GPIO.split();
     // Configure RX and TX pins accordingly
     let tx = gpio.pin24.into_push_pull_output().downgrade();
     let rx = gpio.pin25.into_floating_input().downgrade();