HACKING.rst: Remove -k flag for individual tests as this is not required

HACKING.rst

@@ -465,12 +465,12 @@ If you want to run a specific test or a group of tests, you
 can specify a prefix to match. E.g. if you want to run all of
 the frontend tests you can do::
 
-  ./setup.py test --addopts '-k tests/frontend/'
+  ./setup.py test --addopts 'tests/frontend/'
 
 Specific tests can be chosen by using the :: delimeter after the test module.
 If you wanted to run the test_build_track test within frontend/buildtrack.py you could do::
 
-  ./setup.py test --adopts '-k tests/frontend/buildtrack.py::test_build_track'
+  ./setup.py test --addopts 'tests/frontend/buildtrack.py::test_build_track'
 
 We also have a set of slow integration tests that are disabled by
 default - you will notice most of them marked with SKIP in the pytest