Update settings.md
Update settings.md

We shouldn't propose people to use incorrectly named env variables:
previously:
if "SHOW_DRFDOCS" is True - docs are hidden
now: correct naming of variables.
ekonstantinidis/django-rest-framework-docs,ekonstantinidis/django-rest-framework-docs,manosim/django-rest-framework-docs,manosim/django-rest-framework-docs,ekonstantinidis/django-rest-framework-docs,manosim/django-rest-framework-docs