[FIX] project: fixed stat button in project.project form

Steps to Reproduce:

- Install Project App
- Go inside any  project and set the state of task as done
- Click on project burger menu and click on settings

Issue:

- The count on the stat button refers to count of all tasks

Causes:

- Reference of wrong field in the XML i.e ., 'task_count' instead of
 'open_task_count'.

Solution:

- Changing the field to reference it to open tasks count i.e., 'open_task_count'

task-3741260

closes odoo/odoo#153984

Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>

addons/project/views/project_project_views.xml

@@ -44,7 +44,7 @@
                 <sheet string="Project">
                     <div class="oe_button_box" name="button_box" groups="base.group_user">
                         <button class="oe_stat_button" type="object" name="action_view_tasks" icon="fa-tasks">
-                            <field string="Tasks" name="task_count" widget="statinfo"/>
+                            <field string="Tasks" name="open_task_count" widget="statinfo"/>
                         </button>
                         <button class="oe_stat_button" name="project_update_all_action" type="object" groups="project.group_project_user">
                             <field name="last_update_color" invisible="1"/>