docs: Fix typo: anique → aunique
docs: Fix typo: anique → aunique

Reported by M1dgard on IRC

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