Remove semicolon after #[entry]

Otherwise you get the following build error:

```
error: expected item after attributes
 --> src/main.rs:8:8
  |
8 | #[entry];
  |        ^
```

This one confused me for awhile.

src/getting-started/01.00.BUILD.md

@@ -240,7 +240,7 @@ extern crate panic_halt;
 
 use cortex_m_rt::entry;
 
-#[entry];
+#[entry]
 fn main() {
 }
 ```