Remove unused mut

`gpio` does not need to be mutable. Remove `mut` to silence build
warning.

src/hello-world/02.00.UART.md

@@ -30,7 +30,7 @@ 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();