[1.8.x] Fixed typo in docs/internals/deprecation.txt

* ``django.db.backends.DatabaseValidation.validate_field`` will be removed in

  favor of the ``check_field`` method.

* The ``check`` management command will be removed.

* The ``validate`` management command will be removed.

* ``django.utils.module_loading.import_by_path`` will be removed in favor of

  ``django.utils.module_loading.import_string``.