[FIX] uom: rename ratio field

The ratio field in the UoM form view has 2 different labels depending
on the type (bigger or smaller). This commit put the same label 'ratio'
for two cases.

Task :1859328

addons/uom/views/uom_uom_views.xml

@@ -27,6 +27,7 @@
                             attrs="{'invisible':[('uom_type','!=','smaller')],
                                     'readonly':[('uom_type','=','bigger')]}"/>
                         <field name="factor_inv"
+                            string="Ratio"
                             digits="[42,5]"
                             attrs="{'invisible':[('uom_type','!=','bigger')],
                                     'readonly':[('uom_type','!=','bigger')]}"/>