write_fmt() method needed for write!() macro

Include `use core::fmt::Write;` in serial example.

src/hello-world/02.00.UART.md

@@ -24,6 +24,7 @@ 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;