zephyr: printk: Fix doc error

The docs reference `alloc::format`.  Include the extern for this crate
so that this will work.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

zephyr/src/printk.rs

@@ -5,6 +5,9 @@
 //!
 //! This uses the `k_str_out` syscall, which is part of printk to output to the console.
 
+// Needed by the docs, even though we don't allocate.
+extern crate alloc;
+
 use core::fmt::{
     Arguments,
     Result,