docs(material-input): update usage

In [demo site](https://material.angularjs.org/#/material.components.form/directive/materialInput), the usage of `material-input` has some mistake.

Wrong:
```html
<material-input-group>
  <material-input type="text" ng-model="user.fullName">
  <material-input type="text" ng-model="user.email">
</material-input></material-input></material-input-group>
```

Right:
```html
<material-input-group>
  <material-input type="text" ng-model="user.fullName"></material-input>
  <material-input type="text" ng-model="user.email"></material-input>
</material-input-group>
```