test: updated expected Vec src path

src/test/ui/issues/issue-20433.stderr

@@ -4,7 +4,7 @@ error[E0277]: the size for values of type `[i32]` cannot be known at compilation
 LL |     fn iceman(c: Vec<[i32]>) {}
    |                  ^^^^^^^^^^ doesn't have a size known at compile-time
    | 
-  ::: $SRC_DIR/alloc/src/vec.rs:LL:COL
+  ::: $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
    |
 LL | pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
    |                - required by this bound in `Vec`