Fix `list -af` example in documentation
Fix `list -af` example in documentation

Didn't work since $tracktotal is not an Album field

sampsyo/beets,jackwilsdon/beets,beetbox/beets,shamangeorge/beets,jackwilsdon/beets,beetbox/beets,sampsyo/beets,shamangeorge/beets,sampsyo/beets,jackwilsdon/beets,ibmibmibm/beets,beetbox/beets,ibmibmibm/beets,shamangeorge/beets,ibmibmibm/beets,ibmibmibm/beets,beetbox/beets,shamangeorge/beets,sampsyo/beets,jackwilsdon/beets