projector: fix label/color legend (tensorflow#5974)

## Motivation for features / changes

Inside the projector plugin, a legend for labels/colors should appear
when selecting a field from the "Color by" drop-down menu yet is not
displayed.

## Technical description of changes

The `dom-if` property is misconfigured for the `vz-projector-legend`
template.

Co-authored-by: Marco Romanelli <[email protected]>

Author: 2 people

tensorboard/plugins/projector/vz_projector/vz-projector-data-panel.html.ts

@@ -347,7 +347,7 @@ export const template = html`
           coloring by a given metadata field.
         </paper-tooltip>
       </div>
-      <template dom-if="[[colorLegendRenderInfo]]">
+      <template is="dom-if" if="[[colorLegendRenderInfo]]">
         <vz-projector-legend
           render-info="[[colorLegendRenderInfo]]"
         ></vz-projector-legend>