Docs: fix stestr run example

In TESTING.rst, the `stestr run` incorrectly used `-t` to pass a test
filter. The option is common to `stestr` and is used to specify the test
path used for unittest discovery. It is already configured in
`.stestr.conf`:

  [DEFAULT]
  test_path=tests/unit

One just has to pass the fully qualified test name as an argument.

Change-Id: I8086ba862e5140eb2e1134c96359f1f30032b198

TESTING.rst

@@ -93,7 +93,7 @@ For example, to *run a single Zuul test*::
 To *run one test in the foreground* (after previously having run tox
 to set up the virtualenv)::
 
-  .tox/py35/bin/stestr run -t tests.unit.test_scheduler.TestScheduler.test_jobs_executed
+  .tox/py35/bin/stestr run tests.unit.test_scheduler.TestScheduler.test_jobs_executed
 
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