Replace panic_implementation with panic_handler in example

Author: Martin Larralde

examples/hello_rust_world/src/lib.rs

@@ -1,5 +1,5 @@
 #![allow(dead_code, unused_imports, unused_variables, unused_macros, unused_parens)]
-#![feature(lang_items, core_intrinsics, panic_implementation, start, used, const_fn)]
+#![feature(lang_items, core_intrinsics, panic_handler, start, used, const_fn)]
 #![no_std]
 
 extern crate psp2_sys as psp2;
@@ -14,7 +14,7 @@ use core::panic::PanicInfo;
 #[no_mangle]
 pub extern "C" fn eh_personality() {}
 
-#[panic_implementation]
+#[panic_handler]
 #[no_mangle]
 fn panic(_info: &PanicInfo) -> ! {
     unsafe { intrinsics::abort() }