Update docs/usage.rst
Update docs/usage.rst

I think it should be ``wkhtmltopdf`` and not ``django_wkhtmltopdf``
powderflask/django-wkhtmltopdf,unrealsolver/django-wkhtmltopdf,halfnibble/django-wkhtmltopdf,halfnibble/django-wkhtmltopdf,tclancy/django-wkhtmltopdf,fankcoder/django-wkhtmltopdf,fankcoder/django-wkhtmltopdf,denisbalyko/django-wkhtmltopdf,tclancy/django-wkhtmltopdf,powderflask/django-wkhtmltopdf,incuna/django-wkhtmltopdf,denisbalyko/django-wkhtmltopdf,unrealsolver/django-wkhtmltopdf,incuna/django-wkhtmltopdf