Fix wording of sys.exit docstring. Close SF bug 534113.

Python/sysmodule.c

@@ -156,7 +156,7 @@ static char exit_doc[] =
 \n\
 Exit the interpreter by raising SystemExit(status).\n\
 If the status is omitted or None, it defaults to zero (i.e., success).\n\
-If the status numeric, it will be used as the system exit status.\n\
+If the status is numeric, it will be used as the system exit status.\n\
 If it is another kind of object, it will be printed and the system\n\
 exit status will be one (i.e., failure).";