use #[entry] instead of entry!

>[breaking-change] the entry!, pre_init! and exception! macros have been replaced with attributes: #[entry], #[pre_init] and #[exception], respectively.

9a4a260398/CHANGELOG.md (v060---2018-09-06)

src/getting-started/03.00.DEBUG.md

@@ -6,7 +6,7 @@ Before we start, let's add some code to debug:
 
 ``` rust
 // -- snip --
-entry!(main);
+#[entry]
 fn main() -> ! {
     let _y;
     let x = 42;