error in installation instructions

The installation instructions at the following page https://adoptium.net/installation/linux/ for Deb (Ubuntu) Linux appear to have a step missing.

The following error occurs:

```
$ sudo echo "deb [signed-by=/etc/apt/keyrings/adoptium.asc] https://packages.adoptium.net/artifactory/deb $(awk -F= '/^UBUNTU_CODENAME/{print$2}' /etc/os-release) main" | sudo tee /etc/apt/sources.list.d/adoptium.list
deb [signed-by=/etc/apt/keyrings/adoptium.asc] https://packages.adoptium.net/artifactory/deb focal main
$ sudo apt update
```
```

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.adoptium.net/artifactory/deb focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 843C48A565F8F04B
W: Failed to fetch https://packages.adoptium.net/artifactory/deb/dists/focal/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 843C48A565F8F04B
W: Some index files failed to download. They have been ignored, or old ones used instead.
```

Start at the linked Adoptium Linux installation page and reproduce the documented commands on Ubuntu/Debian, focusing on the missing repository-key setup. Compare the instructions with the NO_PUBKEY error; the work is done when a reader can follow the page and run apt update without the signature-verification failure.

Written by the indexing model from the issue text.