[IMP] sale: changed default sequence prefix of order

There is a confusion with SO when it comes to payment ref. 'SO001' is too
visually to close to 'S0001'.

And wrong payment ref don't help the user reconciling the payments.
So, changed the prefix in default sequence of sales orders from 'SO' to 'S'.

task-2033229

addons/sale/data/ir_sequence_data.xml

@@ -6,7 +6,7 @@
         <record id="seq_sale_order" model="ir.sequence">
             <field name="name">Sales Order</field>
             <field name="code">sale.order</field>
-            <field name="prefix">SO</field>
+            <field name="prefix">S</field>
             <field name="padding">3</field>
             <field name="company_id" eval="False"/>
         </record>

addons/website_sale/static/tests/tours/website_sale_shop_mail.js

@@ -94,7 +94,7 @@ tour.register('shop_mail', {
     },
     {
         content: "click on the first email",
-        trigger: '.o_data_cell:contains("(Ref SO")',
+        trigger: '.o_data_cell:contains("(Ref S")',
     },
     {
         content: "check it's the correct email, and the URL is correct too",