No need of instructions to install requirements for tests

[Pravin]

​https://docs.djangoproject.com/en/dev/intro/contributing/#running-django-s-test-suite-for-the-first-time
Currently we have this:
"Before running the test suite, enter the Django tests/ directory using the cd tests command, and install test dependencies by running"
python -m pip install -r requirements/py3.txt

But this could be have been more clear and straightforward. instead telling user to enter into tests folder we could simply say:
"Before running the test suite, install test dependencies by running:""
python -m pip install -r tests/requirements/py3.txt

This would be more clearer and straightforward.

[Jacob Walls]

Thanks for the idea. This would require also rewriting the section below that shows running ./runtests.py. I don't think it's worth it.