[3.6] bpo-31201: Clarify command to re-run failing test(s) with example (GH-6417) (GH-6438)

(cherry picked from commit 0f914b5b)
Co-authored-by: Aaron Ang <aaronang@users.noreply.github.com>

README.rst

@@ -171,9 +171,10 @@ something is wrong.
 By default, tests are prevented from overusing resources like disk space and
 memory.  To enable these tests, run ``make testall``.
 
-If any tests fail, you can re-run the failing test(s) in verbose mode::
+If any tests fail, you can re-run the failing test(s) in verbose mode.  For
+example, if ``test_os`` and ``test_gdb`` failed, you can run::
 
-    make test TESTOPTS="-v test_that_failed"
+    make test TESTOPTS="-v test_os test_gdb"
 
 If the failure persists and appears to be a problem with Python rather than
 your environment, you can `file a bug report <https://bugs.python.org>`_ and