Fix typo in purchase change unit of measure error message

modules/purchase/message.xml

@@ -4,7 +4,7 @@ this repository contains the full copyright notices and license terms. -->
 <tryton>
     <data grouped="1">
         <record model="ir.message" id="msg_change_purchase_uom">
-            <field name="text">You are trying to change the purchase unit of measurer on which the purchase prices are based.</field>
+            <field name="text">You are trying to change the purchase unit of measure on which the purchase prices are based.</field>
         </record>
         <record model="ir.message" id="msg_erase_party_pending_purchase">
             <field name="text">You cannot erase party "%(party)s" while they have pending purchases with company "%(company)s".</field>