[FIX] developer/tutorials: Button example without deprecated attrs

Using attrs to make a button invisible is deprecated since 17.0 but the button
example hyperlink sends to a commit that used it so it induces people following
the tutorial to use attrs which shouldn't be done anymore.

This commit replaces the link with the commit that replaced the attrs="" to the
plain invisible="" use on the exact same button. (And adjusts the size of the
heading delimiter to respect documentation guidelines.)

Author: Megaaaaaa

content/developer/tutorials/server_framework_101/09_actions.rst

@@ -1,6 +1,6 @@
-==================================
+=================================
 Chapter 9: Ready For Some Action?
-==================================
+=================================
 
 So far we have mostly built our module by declaring fields and views. We just introduced business
 logic in the :doc:`previous chapter <08_compute_onchange>` thanks to
@@ -93,7 +93,7 @@ Finally, a public method should always return something so that it can be called
 When in doubt, just ``return True``.
 
 There are hundreds of examples in the Odoo source code. One example is this
-`button in a view <https://github.com/odoo/odoo/blob/cd9af815ba591935cda367d33a1d090f248dd18d/addons/crm/views/crm_lead_views.xml#L9-L11>`__
+`button in a view <https://github.com/odoo/odoo/blob/55cccddeb12a2096730e4fadc84c52fa838b1aec/addons/crm/views/crm_lead_views.xml#L9-L11>`__
 and its
 `corresponding Python method <https://github.com/odoo/odoo/blob/cd9af815ba591935cda367d33a1d090f248dd18d/addons/crm/models/crm_lead.py#L746-L760>`__