docs(changelog): fill in backfill PR number (#421)

Signed-off-by: Andrei Gherghescu <8067229+andrei-ng@users.noreply.github.com>

Author: jqnatividad

CHANGELOG.md

@@ -14,7 +14,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/) a
 - [[#414](https://github.com/plotly/plotly.rs/issues/414)] Add `DensityMap` (MapLibre `map` subplot) trace type — density heatmaps with full color-scale and hover support
 - [[#417](https://github.com/plotly/plotly.rs/issues/417)] Add `ScatterMap` (MapLibre `map` subplot) trace type — the modern counterpart to `ScatterMapbox`
 - [[#418](https://github.com/plotly/plotly.rs/issues/418)] Add native point clustering to `ScatterMap` via a `Cluster` option
-- [TBD] Backfill common trace attributes on the older trace types for plotly.js 3.7 parity:
+- [[#421](https://github.com/plotly/plotly.rs/pull/421)] Backfill common trace attributes on the older trace types for plotly.js 3.7 parity:
   - `legendrank`, `legendwidth`, `uirevision` on `Scatter`, `Bar`, `BoxPlot`, `Violin`, `Histogram`, `HeatMap`, `Contour`, `Candlestick`, `Ohlc`, `Scatter3D`, `Surface`, `ScatterPolar`, `Sunburst`, `Treemap`, `Table`; `offsetgroup`/`alignmentgroup` on `Scatter`
   - `selected`/`unselected`/`selectedpoints` on `Scatter`, `Bar`, `BoxPlot`, `Histogram` (`selectedpoints` only on `Candlestick`, `Ohlc`)
   - `zorder` on `Scatter`, `Bar`, `BoxPlot`, `Histogram`, `HeatMap`, `Contour`, `Candlestick`, `Ohlc`