text: Title: docs incorrects for 'how to customize templates' section
  Body: Hi there!
  
  There is an error in the docs, specifically in the [how to customize templates section].(https://django-oscar.readthedocs.io/en/2.1.0/howto/how_to_customise_templates.html)
  
  According to issue #1378, the _base.html_ mentioned in the examples should be in '_templates/oscar/base.html_' instead of '_templates/base.html_'.
  
  I'm new to django oscar, and I spent a couple of hours before I could realize what were wrong, so I think it would be very helpful to new devs :)
  
  Regards!
  