doc: Improve documentation for buddy system heap order explanation

- Clarify that the real order of the buddy system is `ORDER - 1`.

Signed-off-by: bigsaltyfishes <[email protected]>

src/lib.rs

@@ -39,7 +39,10 @@ pub use frame::*;
 /// ```
 /// use buddy_system_allocator::*;
 /// # use core::mem::size_of;
-/// let mut heap = Heap::<32>::empty();
+/// // The order of the buddy system is `ORDER - 1`.
+/// // For example, to create a heap with a maximum block size of 2^32 bytes,
+/// // you should define the heap with `ORDER = 33`.
+/// let mut heap = Heap::<33>::empty();
 /// # let space: [usize; 100] = [0; 100];
 /// # let begin: usize = space.as_ptr() as usize;
 /// # let end: usize = begin + 100 * size_of::<usize>();