Missing dropdown for column selection mode in Column Selection Tree grid topic

[valadzhov]

1. Open https://staging.infragistics.com/products/ignite-ui-blazor/blazor/components/grids/tree-grid/column-selection
2. Examine the text under the Blazor Tree Grid Column Selection Example -> "The sample below demonstrates the three types of IgbTreeGrid's column selection behavior. Use the column selection dropdown below to enable each of the available selection modes."
3. Examine the sample -> there's no dropdown for column selection mode 😑