docs(material/select): fix select within dialog example not selecting option

Author: annieyw

src/dev-app/select/select-demo.html

@@ -261,9 +261,9 @@
         <mat-form-field>
           <mat-label>Select a topping</mat-label>
           <mat-select>
-            <mat-option>Cheese</mat-option>
-            <mat-option>Onion</mat-option>
-            <mat-option>Pepper</mat-option>
+            <mat-option value="cheese">Cheese</mat-option>
+            <mat-option value="onion">Onion</mat-option>
+            <mat-option value="pepper">Pepper</mat-option>
           </mat-select>
         </mat-form-field>