Docs: remove test name example with command line and arguments

While a given resolver could return such a test name, I think this is
a bad example for a Test Name in this section.

The reason is that it's not possible, with the available test
resolvers/loaders to create a test such as that (it used to be).
Signed-off-by: NCleber Rosa <crosa@redhat.com>

docs/source/ReferenceGuide.rst

@@ -111,7 +111,6 @@ Syntax::
 Examples of test-names::
 
    '/bin/true'
-   '/bin/grep foobar /etc/passwd'
    'passtest.py:Passtest.test'
    'file:///tmp/passtest.py:Passtest.test'
    'multiple_tests.py:MultipleTests.test_hello'