slight language tweak in duplicates plugin docs
slight language tweak in duplicates plugin docs

Suggested by @sampsyo in https://github.com/beetbox/beets/pull/2193

jackwilsdon/beets,mosesfistos1/beetbox,MyTunesFreeMusic/privacy-policy,shamangeorge/beets,mosesfistos1/beetbox,SusannaMaria/beets,Kraymer/beets,MyTunesFreeMusic/privacy-policy,sampsyo/beets,jackwilsdon/beets,Kraymer/beets,mosesfistos1/beetbox,lengtche/beets,sampsyo/beets,pkess/beets,sampsyo/beets,jackwilsdon/beets,madmouser1/beets,lengtche/beets,lengtche/beets,shamangeorge/beets,madmouser1/beets,shamangeorge/beets,pkess/beets,xsteadfastx/beets,ibmibmibm/beets,sampsyo/beets,SusannaMaria/beets,xsteadfastx/beets,pkess/beets,mosesfistos1/beetbox,artemutin/beets,ibmibmibm/beets,diego-plan9/beets,xsteadfastx/beets,madmouser1/beets,beetbox/beets,artemutin/beets,diego-plan9/beets,artemutin/beets,diego-plan9/beets,beetbox/beets,SusannaMaria/beets,lengtche/beets,diego-plan9/beets,MyTunesFreeMusic/privacy-policy,shamangeorge/beets,jackwilsdon/beets,ibmibmibm/beets,SusannaMaria/beets,MyTunesFreeMusic/privacy-policy,ibmibmibm/beets,pkess/beets,Kraymer/beets,xsteadfastx/beets,artemutin/beets,Kraymer/beets,beetbox/beets,beetbox/beets,madmouser1/beets