fix: toc menu missing plotting name (#591)

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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