copy_nonoverlapping example: Fixed typo

The comment referred to a variable using an incorrect name. (it has probably been renamed since the comment was written, or the comment was copied elsewhere - I noted the example in libcore has the `tmp` name for the temporary variable.)

src/libcore/intrinsics.rs

@@ -992,7 +992,7 @@
     ///         ptr::copy_nonoverlapping(y, x, 1);
     ///         ptr::copy_nonoverlapping(&t, y, 1);
     ///
-    ///         // y and t now point to the same thing, but we need to completely forget `tmp`
+    ///         // y and t now point to the same thing, but we need to completely forget `t`
     ///         // because it's no longer relevant.
     ///         mem::forget(t);
     ///     }