Improve as_mut ptr method example

Author: GuillaumeGomez

src/libcore/ptr.rs

@@ -459,6 +459,9 @@ impl<T: ?Sized> *mut T {
     /// ```
     /// let mut s = [1, 2, 3];
     /// let ptr: *mut u32 = s.as_mut_ptr();
+    /// let first_value = unsafe { ptr.as_mut().unwrap() };
+    /// *first_value = 4;
+    /// println!("{:?}", s); // It'll print: "[4, 2, 3]".
     /// ```
     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
     #[inline]