Update README `keyword` usage (#1044)
Update README `keyword` usage (#1044)

to the current ES version according to https://github.com/ankane/searchkick/blob/61ede188042c843c1dfd15fc724379ce69353887/test/test_helper.rb#L457

Otherwise, it's confusing
spanner/searchkick,mikelkew/searchkick,johan--/searchkick,mikelkew/searchkick,spanner/searchkick,johan--/searchkick,ankane/searchkick