thank you Medhi for your answer,
I can see my mistake, instead of:
<a href="/<?php echo $formation->link; ?>"><?php echo $formation->value; ?></a>
I just have to use:
<?php echo $formation->text; ?>
or
<?php echo $formation->html; ?>
the link is already in the variable, that mean than I can't customise it here...
I'm going to try what I saw in another topic, about using List & type to only have one url by page, it seems to be the only way.