zalom said almost 3 years ago on A Rubyist's Apple M1 Review :
There is a missing `\n` in this line ->
echo ". $(brew --prefix asdf)/asdf.sh" >> ~/.zshrc

It should look like:

echo -e "\n. $(brew --prefix asdf)/asdf.sh" >> ~/.zshrc

Also, as per this comment -> https://github.com/asdf-vm/asdf-ruby/issues/81#issuecomment-558234779
I had to add this too ->

echo -e "\n. $(brew --prefix asdf)/etc/bash_completion.d/asdf.bash" >> ~/.zshrc