fix comment in function parameter drop scope example

In "Scopes of function parameters" under "Destructors", the first
comment in the example that demonstrates function parameter
drop order contradicted both the text and the program behavior.

Author: tlyu

src/destructors.md

@@ -110,7 +110,7 @@ dropped after any bindings introduced in that parameter's pattern.
 #         println!("drop({})", self.0);
 #     }
 # }
-// Drops the second parameter, then `y`, then the first parameter, then `x`
+// Drops `y`, then the second parameter, then `x`, then the first parameter
 fn patterns_in_parameters(
     (x, _): (PrintOnDrop, PrintOnDrop),
     (_, y): (PrintOnDrop, PrintOnDrop),