List category posts alphabetically on archive.php

I suggest you use the pre_get_posts action hook provided by WordPress.

Then, in your functions.php add the following code below:

function custom_pre_get_posts($query) {
    // validate
    if(!is_admin() && $query->is_main_query()) {

        if(is_archive()) {
            $query->set('orderby', 'title'); // order posts by title
            $query->set('order', 'ASC'); // and in ascending order
        }
    }
}
add_action('pre_get_posts', 'custom_pre_get_posts');

You can learn more about pre_get_posts in here