fix: toc menu missing plotting name (#591)

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Fixes internal  #333374239 🦕

Author: ashleyxuu

docs/templates/toc.yml

@@ -69,6 +69,7 @@
         uid: bigframes.operations.plotting
       - name: PlotAccessor
         uid: bigframes.operations.plotting.PlotAccessor
+      name: Plotting
     - items:
       - name: Series
         uid: bigframes.series.Series