fix(material/table): using columnsToDisplay length for colspan attribute

Author: forsti0506

src/components-examples/material/table/table-expandable-rows/table-expandable-rows-example.html

@@ -17,7 +17,7 @@
 
   <!-- Expanded Content Column - The detail row is made up of this one column that spans across all columns -->
   <ng-container matColumnDef="expandedDetail">
-    <td mat-cell *matCellDef="let element" [attr.colspan]="columnsToDisplay.length">
+    <td mat-cell *matCellDef="let element" [attr.colspan]="columnsToDisplayWithExpand.length">
       <div class="example-element-detail"
            [@detailExpand]="element == expandedElement ? 'expanded' : 'collapsed'">
         <div class="example-element-diagram">