docs: Add `apt update` command before installing `aem`

[Christewart]

If I don't do an sudo apt update after adding the repo and key I get this error

sudo apt install apollo-neo-env-manager-dev
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
E: Unable to locate package apollo-neo-env-manager-dev

Adding sudo apt update and then installing apollo-neo-env-manager-dev fixes this on my machine.