Merge pull request urfave#2251 from icholy/duration-flag-trace-message

Fix: use the correct type name in the tracef message

Author: dearchap

flag_duration.go

@@ -45,6 +45,6 @@ func (cmd *Command) Duration(name string) time.Duration {
 		return v
 	}
 
-	tracef("bool NOT available for flag name %[1]q (cmd=%[2]q)", name, cmd.Name)
+	tracef("duration NOT available for flag name %[1]q (cmd=%[2]q)", name, cmd.Name)
 	return 0
 }