Add missing device path to '-l' example

Also, it's ttyS0 not ttySO.

st.1

@@ -96,18 +96,18 @@ use a tty
 .I line
 instead of a pseudo terminal.
 .I line
-should be a (pseudo-)serial device (e.g. /dev/ttySO on Linux for serial port
+should be a (pseudo-)serial device (e.g. /dev/ttyS0 on Linux for serial port
 0).
 When this flag is given
 remaining arguments are used as flags for
 .BR stty(1).
 By default st initializes the serial line to 8 bits, no parity, 1 stop bit
 and a 38400 baud rate. The speed is set by appending it as last argument
-(e.g. 'st -l 115200'). Arguments before the last one are
+(e.g. 'st -l /dev/ttyS0 115200'). Arguments before the last one are
 .BR stty(1)
 flags. If you want to set odd parity on 115200 baud use for example 'st -l
-parenb parodd 115200'. Set the number of bits by using for example 'st -l cs7
+/dev/ttyS0 parenb parodd 115200'. Set the number of bits by using for
-115200'. See
+example 'st -l /dev/ttyS0 cs7 115200'. See
 .BR stty(1)
 for more arguments and cases.
 .TP