let return in format->cells: format-code entry widget activate ok

Change-Id: Ibeb51624a12a62d9f93d1b1f712cbdde83cce2c7
Reviewed-on: https://gerrit.libreoffice.org/66727
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

cui/uiconfig/ui/numberingformatpage.ui

@@ -82,6 +82,7 @@
                     <property name="can_focus">True</property>
                     <property name="valign">center</property>
                     <property name="hexpand">True</property>
+                    <property name="activates_default">True</property>
                   </object>
                   <packing>
                     <property name="left_attach">0</property>