Fixed typo in POTS definition.

docs/sexpots.txt

@@ -232,7 +232,7 @@ KBPS    = One thousand Bits Per Second
 IP      = Internet Protocol
 Modem   = Modulator / Demodulator
 NT      = Windows NT (New Technology), any of Windows NT/2000/XP/2003/Vista OS
-POTS    = Plain Old Telephone System/System
+POTS    = Plain Old Telephone Service/System
 Sysop   = System Operator (of a BBS)
 Telnet  = Teletype Network protocol (over TCP, see RFC 854)
 TCP     = Transmission Control Protocol (over IP)