[1.7.x] Fixed typo in docs/topics/i18n/translation.txt

dictionary under that key during string interpolation. Here's example::

    from django import forms

    from django.utils.translation import ugettext_lazy

    from django.utils.translation import ungettext_lazy

    class MyForm(forms.Form):

        error_message = ungettext_lazy("You only provided %(num)d argument",