[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.)

closes #18047

X-original-commit: 05c46d7
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>
Signed-off-by: Thomas Becquevort (thbe) <thbe@odoo.com>

Author: Megaaaaaa

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

@@ -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>`__