Fix grammar typo in runtest not-annotation help.

Change-Id: I5f5a81d16715f968ed9b265398d2a9e483c23a89
diff --git a/testrunner/runtest.py b/testrunner/runtest.py
index 41063f1..d53312f 100755
--- a/testrunner/runtest.py
+++ b/testrunner/runtest.py
@@ -124,7 +124,7 @@
                       help="Include only those tests tagged with a specific"
                       " annotation")
     parser.add_option("--not-annotation", dest="test_not_annotation",
-                      help="Excludes any tests tagged with a specific"
+                      help="Exclude any tests tagged with a specific"
                       " annotation")
     parser.add_option("-u", "--user-tests-file", dest="user_tests_file",
                       metavar="FILE", default=user_test_default,