Fix the example linked list possibly dropping an uninitalized element

It even says it in the safety comment... smh my head.

src/lib.rs

@@ -67,7 +67,7 @@ use core::{
 ///
 ///         // SAFETY: `SlabAllocator::allocate` gives out pointers that are valid for writes (but
 ///         // **not** for reads).
-///         unsafe { *node.as_ptr() = Node::new(value) };
+///         unsafe { ptr::write(node.as_ptr(), Node::new(value)) };
 ///
 ///         if let Some(tail) = self.tail {
 ///             unsafe { (*tail.as_ptr()).next = Some(node) };