Update docs for 0.16.0 update of asdf Golang version

[Skeeg]

No description provided.

[timkinnane]

This looks good @Skeeg, thanks for contributing.

I made a similar change locally just a couple days ago after finding that sourcing asdf.sh hasn't been required for quite a while.

I left the environment file loading though because I had to add some other values to get plugins working, e.g:

export ASDF_DIR="$HOME/.asdf"
export ASDF_DATA_DIR="$HOME/.asdf"
export PATH="${ASDF_DATA_DIR:-$HOME/.asdf}/shims:$PATH"
export ASDF_CONCURRENCY=auto

I'm not sure any of that is actually necessary though, or just a problem with the Node plugin version.

I'd be okay with your changes as is for now if you take them out of draft, then we can see about moving to Mise as a follow up. It does seem a lot easier to setup and maintain so far.