Update rigctld.1 man page to add -1 for no passband change

doc/man1/rigctld.1

@@ -474,7 +474,7 @@ Mode is a token: \(oqUSB\(cq, \(oqLSB\(cq, \(oqCW\(cq, \(oqCWR\(cq,
 \(oqECSSLSB\(cq, \(oqFA\(cq, \(oqSAM\(cq, \(oqSAL\(cq, \(oqSAH\(cq,
 \(oqDSB\(cq.
 .IP
-Passband is in Hz as an integer, or \(oq0\(cq for the radio backend default.
+Passband is in Hz as an integer, -1 for no change, or \(oq0\(cq for the radio backend default.
 .IP
 .BR Note :
 Passing a \(oq?\(cq (query) as the first argument instead of a Mode token will