Fixed typo in contrib.rst ("pipeline" -> "_pipeline")

There is a typo on https://django-jinja.readthedocs.org/en/latest/contrib.html:

    INSTALLED_APPS += ('django_jinja.contrib.pipeline',)

should be

    INSTALLED_APPS += ('django_jinja.contrib._pipeline',)

Also, this is inconsistent with the example above the code block.