Clarify that print_something needs to be called from _start

blog/content/second-edition/posts/03-vga-text-buffer/index.md

@@ -255,7 +255,20 @@ It first creates a new Writer that points to the VGA buffer at `0xb8000`. The sy
 [raw pointer]: https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer
 [`unsafe` block]: https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html
 
-Then it writes the byte `b'H'` to it. The `b` prefix creates a [byte literal], which represents an ASCII character. By writing the strings `"ello "'` and `"Wörld!"`, we test our `write_string` method and the handling of unprintable characters.  When we call `vga_buffer::print_something` in our `_start` function (in `src/main.rs`), a `Hello W■■rld!` should be printed in the _lower_ left corner of the screen in yellow:
+Then it writes the byte `b'H'` to it. The `b` prefix creates a [byte literal], which represents an ASCII character. By writing the strings `"ello "'` and `"Wörld!"`, we test our `write_string` method and the handling of unprintable characters. To see the output, we need to call the `printt_something` function from our `_start` function:
+
+```rust
+// in src/main.rs
+
+#[no_mangle]
+pub extern "C" fn _start() -> ! {
+    vga_buffer::print_something();
+
+    loop {}
+}
+```
+
+When we run our project now, a `Hello W■■rld!` should be printed in the _lower_ left corner of the screen in yellow:
 
 [byte literal]: https://doc.rust-lang.org/reference/tokens.html#byte-literals