Fix error in archives.blade.php

Change "index" action by "indexByYearMonth" action

So we get:
<a href="{{ action('Fbf\LaravelBlog\PostsController@indexByYearMonth', array('year' => $year, 'month' => $monthNumber)) }}">

instead of:
<a href="{{ action('Fbf\LaravelBlog\PostsController@index', array('year' => $year, 'month' => $monthNumber)) }}">