Fix links to error codes

Author: GuillaumeGomez

library/core/src/marker.rs

@@ -343,7 +343,7 @@ pub trait StructuralEq {
 /// If you try to implement `Copy` on a struct or enum containing non-`Copy` data, you will get
 /// the error [E0204].
 ///
-/// [E0204]: ../../error-index.html#E0204
+/// [E0204]: ../../error_codes/E0204.html
 ///
 /// ## When *should* my type be `Copy`?
 ///

library/core/src/ops/drop.rs

@@ -156,7 +156,7 @@ pub trait Drop {
     /// handled by the compiler, but when using unsafe code, can sometimes occur
     /// unintentionally, particularly when using [`ptr::drop_in_place`].
     ///
-    /// [E0040]: ../../error-index.html#E0040
+    /// [E0040]: ../../error_codes/E0040.html
     /// [`panic!`]: crate::panic!
     /// [`mem::drop`]: drop
     /// [`ptr::drop_in_place`]: crate::ptr::drop_in_place