channeldb+lnwallet: note that balance is after subtracting commit fee

It was incorrectly stated that the commitment balance was before
subctracting the commit fee, which led to some confusion.

lnwallet/channel.go

@@ -502,7 +502,7 @@ type commitment struct {
 	// evaluating all the add/remove/settle log entries before the listed
 	// indexes.
 	//
-	// NOTE: This is the balance *before* subtracting any commitment fee.
+	// NOTE: This is the balance *after* subtracting any commitment fee.
 	ourBalance   lnwire.MilliSatoshi
 	theirBalance lnwire.MilliSatoshi