[IMP] purchase: rename Invoices button to Vendor Bills

As this button redirects to the Vendor Bills page, let us rename this
button so that names match.

addons/purchase/views/purchase_views.xml

@@ -182,7 +182,7 @@
                         <button type="object"  name="action_view_invoice"
                             class="oe_stat_button"
                             icon="fa-pencil-square-o" attrs="{'invisible':[('state', 'in', ('draft','sent','to approve')),('invoice_ids','=',[])]}">
-                            <field name="invoice_count" widget="statinfo" string="Invoices"/>
+                            <field name="invoice_count" widget="statinfo" string="Vendor Bills"/>
                             <field name='invoice_ids' invisible="1"/>
                         </button> 
                     </div>