update the unsafety example

Author: japaric

cortex-m-rt/examples/unsafety.rs

@@ -5,7 +5,7 @@
 #![no_std]
 
 extern crate cortex_m_rt;
-extern crate panic_semihosting;
+extern crate panic_halt;
 
 use cortex_m_rt::{entry, exception, ExceptionFrame};