Просто поменяйте XPath на пробег
от
mileage = advert.xpath( './/*[@class="specs"]//li[1]/text()' ).extract()
до
mileage = advert.xpath( './/*[@class="specs"]//li[1]/text()[2]' ).extract()
Вы получите правильный вывод:
title,price,mileage
ALFA ROMEO GIULIA (0) V6 BITURBO QUADRIFOGLIO 2018 (2018),"£48,500",5500
ULEZ CHARGE EXEMPT! EURO 6 (2017),"£25,695",11450
ALFA ROMEO GIULIA (0) V6 BITURBO QUADRIFOGLIO NRING 2019 (2019),"£83,500",100
ALFA ROMEO GIULIA (0) TD SPECIALE 2017 (2017),"£22,500",23700