Describe the bug
It appears that the command:

apt install -y wget

should be added to the section of the "Prerequisites: Ubuntu (one-off)" documentation.

To Reproduce

1. Follow the installation instructions for Ubuntu as written.
2. When you get to the command spack external find --scope system wget, in the instructions, nothing is found.

Expected behavior
wget should be found on Ubuntu when the spack external find command is run.

System:
An Ubuntu 22.04 VM running on MacOS