gps-log: fix typo on man page. -f is really -l

docs/includes/gps-log-body.txt

@@ -67,7 +67,7 @@ GPS always output a static TDOP.
   This the simplest use of this program. It can be used to check the
   status of the local gpsd daemon.
 
-+gps-log -f /var/log/ntpstats/gpsd -w 60+ &::
++gps-log -l /var/log/ntpstats/gpsd -w 60+ &::
   This will continuously log the gpsd data in the background to the file
   /var/log/ntpstats/gpsd.  Only log every 60 seconds.