[FIX] sale_timesheet: clarify field label

Was unclear
Courtesy of the translators team

addons/sale_timesheet/i18n/sale_timesheet.pot

@@ -171,7 +171,7 @@ msgstr ""
 
 #. module: sale_timesheet
 #: model:ir.model.fields,field_description:sale_timesheet.field_sale_order_line_is_service
-msgid "Is service"
+msgid "Is a Service"
 msgstr ""
 
 #. module: sale_timesheet

addons/sale_timesheet/models/sale_order.py

@@ -136,7 +136,7 @@ class SaleOrderLine(models.Model):
     _inherit = "sale.order.line"
 
     task_id = fields.Many2one('project.task', 'Task', help="Task generated by the sales order item")
-    is_service = fields.Boolean("Is service", compute='_compute_is_service', store=True, help="Sales Order item should generate a task and/or a project, depending on the product settings.")
+    is_service = fields.Boolean("Is a Service", compute='_compute_is_service', store=True, help="Sales Order item should generate a task and/or a project, depending on the product settings.")
 
     @api.multi
     @api.depends('product_id.type')