[FIX] odoo_sh: @api.multi doesn't exist in 13.0 (#747)

`api.multi` was removed in 13.0 and is the default behavior. Attempting
to use it in this tutorial will result in a build error.

odoo_sh/getting_started/first_module.rst

@@ -477,7 +477,6 @@ Add
           values['name'] = unidecode(values['name'])
       return super(my_module, self).create(values)
 
-  @api.multi
   def write(self, values):
       if 'name' in values:
           values['name'] = unidecode(values['name'])