doc: add missing label
doc: add missing label

It suppresses the following warning:

    doc/source/reference/normalizers.txt:21: WARNING: undefined label: table-create-normalizer (if the link has no caption the label must precede a section header)

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