Fix error in archives.blade.php

Archives don't work properly because we have to change "index" action by "indexByYearMonth" action

So we get:
```php
<a href="{{ action('Fbf\LaravelBlog\PostsController@indexByYearMonth', array('year' => $year, 'month' => $monthNumber)) }}">
```
instead of:
```php
<a href="{{ action('Fbf\LaravelBlog\PostsController@index', array('year' => $year, 'month' => $monthNumber)) }}">
```