Fix typo: SIGURS1 should be SIGUSR1

Author: stennie

source/tutorial/rotate-log-files.txt

@@ -12,7 +12,7 @@ Specifically, log rotation renames the current log file by appending
 the filename with a timestamp, [#timestamp-format]_ opens a new log
 file, and finally closes the old log. MongoDB will only rotate logs,
 when you use the :dbcommand:`logRotate` command, or issue the process
-a ``SIGURS1`` signal as described in this procedure.
+a ``SIGUSR1`` signal as described in this procedure.
 
 .. seealso:: For information on logging, see the
    :ref:`monitoring-standard-loggging` section.