Fix comment
Fix comment

It seems that GRN_UVECTOR isn't an array of grn_id. It may be an array
fixed size data such as GRN_DB_UINT64.

cosmo0920/groonga,komainu8/groonga,cosmo0920/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,hiroyuki-sato/groonga,kenhys/groonga,kenhys/groonga,hiroyuki-sato/groonga,kenhys/groonga,groonga/groonga,naoa/groonga,komainu8/groonga,redfigure/groonga,naoa/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,komainu8/groonga,naoa/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,redfigure/groonga,naoa/groonga,redfigure/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,groonga/groonga,cosmo0920/groonga,cosmo0920/groonga,komainu8/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,redfigure/groonga,naoa/groonga,groonga/groonga,komainu8/groonga,redfigure/groonga,naoa/groonga,naoa/groonga,groonga/groonga,naoa/groonga,hiroyuki-sato/groonga,redfigure/groonga,redfigure/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,komainu8/groonga,komainu8/groonga,redfigure/groonga,kenhys/groonga