Update hello-world example for current boilerplate

Author: alex

hello-world/src/lib.rs

@@ -1,5 +1,7 @@
 #![no_std]
-#![feature(alloc, const_str_as_bytes)]
+#![feature(const_str_as_bytes)]
+
+extern crate alloc;
 
 use alloc::borrow::ToOwned;
 use alloc::string::String;