[FIX] hr_contract: employee form statbutton's icon

The name of the icon on the statbutton was apparently broken during the
new-api migration (commit 095d0174fd),
leading to the icon not appearing on the "Contracts" button of the
Employee form.

addons/hr_contract/views/hr_contract_views.xml

@@ -18,7 +18,7 @@
                     <xpath expr="//div[@name='button_box']" position="inside">
                         <button name="%(act_hr_employee_2_hr_contract)d"
                             class="oe_stat_button"
-                            icon="fabook"
+                            icon="fa-book"
                             type="action"
                             groups="base.group_hr_manager">
                             <field name="contracts_count" widget="statinfo" string="Contracts"/>