doc fix (#2676)

fixing error:
```
warning: unresolved link to `well_known_keys::CODE`
   --> pallets/parachain-system/src/lib.rs:560:12
    |
560 |     #[pallet::storage]
    |               ^^^^^^^
    |
    = note: the link appears in this line:
            " As soon as the relay chain gives us the go-ahead signal, we will overwrite the [`:code`][well_known_keys::CODE]"
                                                                                                       ^^^^^^^^^^^^^^^^^^^^^
    = note: no item named `well_known_keys` in scope
    = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
```