From Game Rules > Adversaries > Archer > Open Sheet, it brings up the character sheet for the Archer minion, but the action “Immobilizing Shot” is not visible on the character sheet like it is on the Adversary rule page for this minion.
I just checked this on my end and it appears to be working correctly. Can you try clearing your cache and cookies and then taking another look to see if it shows up correctly for you now?