docs: unresolved link to `well_known_keys::CODE

[lexnv]

After updating Cargo.lock for substrate in the companion: #2655

[2023-05-31 13:00:26] error: unresolved link to `well_known_keys::CODE`
[2023-05-31 13:00:26]    --> pallets/parachain-system/src/lib.rs:560:12
[2023-05-31 13:00:26]     |
[2023-05-31 13:00:26] 560 |     #[pallet::storage]
[2023-05-31 13:00:26]     |               ^^^^^^^
[2023-05-31 13:00:26]     |
[2023-05-31 13:00:26]     = note: the link appears in this line:
[2023-05-31 13:00:26]             
[2023-05-31 13:00:26]             " As soon as the relay chain gives us the go-ahead signal, we will overwrite the [`:code`][well_known_keys::CODE]"
[2023-05-31 13:00:26]                                                                                                        ^^^^^^^^^^^^^^^^^^^^^
[2023-05-31 13:00:26]     = note: no item named `well_known_keys` in scope
[2023-05-31 13:00:26]     = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
[2023-05-31 13:00:26] 
[2023-05-31 13:00:26] error: could not document `cumulus-pallet-parachain-system`

[niklasad1]

The CI should run cargo doc --document-private-items or disable -D warnings IMO