add multiple classes in ngclass code example

Example 1: multiple ngclass

<section [ngClass]="{'class1':condition1, 'class2': condition2, 'class3':condition3}" >

Example 2: ng class multiple class names

<!-- notice expression1 used twice -->
<div ng-class="{class1 : expression1, class2 : expression1}">
    Hello World!
</div>