docs: fix broken gradient table (#5644)
docs: fix broken gradient table (#5644)

part of https://github.com/vega/vega-lite/issues/5634
uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,vega/vega-lite