[1.7.x] Fixed typo in docs/topics/forms/modelforms.txt

values provided this way will override both initial values from the form field

and values from an attached model instance. For example::

    >>> article = Article.objects.get(pk1=)

    >>> article = Article.objects.get(pk=1)

    >>> article.headline

    'My headline'

    >>> form = ArticleForm(initial={'headline': 'Initial headline'), instance=article)