waker context: Fix size and align of raw vtable

src/2_waker_context.md

@@ -98,6 +98,7 @@ Let's explain this in code instead of words by implementing our own trait
 object from these parts:
 
 ```rust
+# use std::mem::{align_of, size_of};
 // A reference to a trait object is a fat pointer: (data_ptr, vtable_ptr)
 trait Test {
     fn add(&self) -> i32;
@@ -139,8 +140,8 @@ fn main() {
     // length of the array is encoded in the array itself as the second value.
     let vtable = vec![
         0,                  // pointer to `Drop` (which we're not implementing here)
-        6,            // length of vtable
-        8,            // alignment
+        size_of::<Data>(),  // length of data
+        align_of::<Data>(), // alignment of data
 
         // we need to make sure we add these in the same order as defined in the Trait.
         add as usize, // function pointer - try changing the order of `add`