Clarify location of `custom.css` file
Clarify location of `custom.css` file

The current documentation doesn't make it clear that the path of `custom.css` file refers to a path within the `files` directory, so when I tried to create a custom CSS file it wasn't picked up.

Not sure if this change is the clearest way of expressing the correct location but it seemed perhaps better than "you can create a ``assets/css/custom.css`` file in the ``files`` directory and then..."
andredias/nikola,getnikola/nikola,x1101/nikola,andredias/nikola,yamila-moreno/nikola,schettino72/nikola,schettino72/nikola,x1101/nikola,andredias/nikola,TyberiusPrime/nikola,TyberiusPrime/nikola,atiro/nikola,lucacerone/nikola,masayuko/nikola,okin/nikola,techdragon/nikola,yamila-moreno/nikola,s2hc-johan/nikola,okin/nikola,JohnTroony/nikola,techdragon/nikola,okin/nikola,lucacerone/nikola,xuhdev/nikola,techdragon/nikola,wcmckee/nikola,berezovskyi/nikola,knowsuchagency/nikola,s2hc-johan/nikola,masayuko/nikola,TyberiusPrime/nikola,xuhdev/nikola,wcmckee/nikola,JohnTroony/nikola,immanetize/nikola,yamila-moreno/nikola,getnikola/nikola,atiro/nikola,xuhdev/nikola,knowsuchagency/nikola,schettino72/nikola,getnikola/nikola,gwax/nikola,gwax/nikola,gwax/nikola,jjconti/nikola,jjconti/nikola,okin/nikola,xuhdev/nikola,berezovskyi/nikola,knowsuchagency/nikola,berezovskyi/nikola,x1101/nikola,pluser/nikola,getnikola/nikola,pluser/nikola,s2hc-johan/nikola,atiro/nikola,masayuko/nikola,lucacerone/nikola,pluser/nikola,immanetize/nikola,wcmckee/nikola,immanetize/nikola,jjconti/nikola,JohnTroony/nikola