Trivial doc fix: text for link to "Testing in Django" section

The bottom of the [Further testing](https://docs.djangoproject.com/en/dev/intro/tutorial05/#further-testing) section of the documentation has a link to "Testing in Django" with the wrong link text.  "Testing Django applications" is a sub-section of ["Testing in Django"](https://docs.djangoproject.com/en/dev/topics/testing/) and is not the name of the testing section as a whole.