tests/rcutorture: update usage hint

Although documented in the comments we don't display all the various
invocations we can in the usage.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200225124710.14152-3-alex.bennee@linaro.org>

tests/rcutorture.c

@@ -413,7 +413,8 @@ static void gtest_stress_10_5(void)
 
 static void usage(int argc, char *argv[])
 {
-    fprintf(stderr, "Usage: %s [nreaders [ perf | stress ] ]\n", argv[0]);
+    fprintf(stderr, "Usage: %s [nreaders [ [r|u]perf | stress [duration]]\n",
+            argv[0]);
     exit(-1);
 }