[FIXED] Fix unsupported MQTT characters error message

Previously ' ' and '.' were not supported for MQTT topics until #4243 brought in support for '.'. However the error message for unsupported characters appears to not have been updated. This simply updates the error message.

Had an example of a MQTT client trying to publish to `spBv1.0/Field Devices/Some Device/Some Topic` and the error message indicated that both '.' and ' ' were unsupported which was misleading as only ' ' is unsupported.

server/mqtt.go

@@ -217,7 +217,7 @@ var (
 	errMQTTEmptyUsername            = errors.New("empty user name not allowed")
 	errMQTTTopicIsEmpty             = errors.New("topic cannot be empty")
 	errMQTTPacketIdentifierIsZero   = errors.New("packet identifier cannot be 0")
-	errMQTTUnsupportedCharacters    = errors.New("characters ' ' and '.' not supported for MQTT topics")
+	errMQTTUnsupportedCharacters    = errors.New("character ' ' not supported for MQTT topics")
 	errMQTTInvalidSession           = errors.New("invalid MQTT session")
 )