Add missing unsafe to an example in the 0.21 migration guide.

docs/0.21-MIGRATION.md

@@ -288,7 +288,7 @@ It's now possible to read and write elements via the `Deref` and `DerefMut` trai
 let byte_array = env.new_byte_array(100)?;
 
 {
-    let mut elements = env.get_array_elements(&byte_array, ReleaseMode::CopyBack)?;
+    let mut elements = unsafe { env.get_array_elements(&byte_array, ReleaseMode::CopyBack) }?;
 
     elements[0] = 0xff;
     assert_eq!(elements[0], 0xff);