Make start and end tag of md-optgroup consistent.

Author: Stexxen

src/components/select/demoOptionGroups/index.html

@@ -4,10 +4,10 @@
     <md-select ng-model="favoriteTopping" placeholder="Favorite Topping">
       <md-optgroup label="Meats">
         <md-option ng-value="topping.name" ng-repeat="topping in toppings | filter: {category: 'meat' }">{{topping.name}}</md-option>
-      </md-option-group>
+      </md-optgroup>
       <md-optgroup label="Veggies">
         <md-option ng-value="topping.name" ng-repeat="topping in toppings | filter: {category: 'veg' }">{{topping.name}}</md-option>
-      </md-option-group>
+      </md-optgroup>
     </md-select>
     <p class="result">{{ favoriteTopping ? 'Your favorite topping is ' + favoriteTopping : 'Please select a topping'}}</p>
   </div>