Update rigctl.1 for IC7300 filter 1,2,3 usage

doc/man1/rigctl.1

@@ -494,6 +494,7 @@ Mode is a token: \(oqUSB\(cq, \(oqLSB\(cq, \(oqCW\(cq, \(oqCWR\(cq,
 \(oqDSB\(cq.
 .IP
 Passband is in Hz as an integer, -1 for no change, or \(oq0\(cq for the radio backend default.
+IC7300 can use 1,2,3 to select which filter to use
 .IP
 .BR Note :
 Passing a \(oq?\(cq (query) as the first argument instead of a Mode token will