doc: point to correct `db` arg for `go get`
doc: point to correct `db` arg for `go get`

per side-chat in https://github.com/restic/restic/issues/494#issuecomment-209591073
restic/restic,ar-jan/restic,arithmetric/restic,mappu/restic,byo/restic,kurin/restic,middelink/restic,stuertz/restic,kurin/restic,byo/restic,fawick/restic,middelink/restic,ckemper67/restic,jayme-github/restic,fawick/restic,restic/restic,jayme-github/restic,stuertz/restic,ar-jan/restic