std::flag: fix compilation error of the add[T] method of the FlagSet

std/flag/flag.jule

@@ -166,7 +166,7 @@ impl FlagSet {
     //  - str
     //  - bool
     pub fn add[T](mut self, name: str, short: rune, default: T, what: str): &T {
-        let mut flag = self.add_flag_common[T](name, short, default, what)
+        let mut flag = self.add_flag_common[T](name, short, what)
         flag._data = new(T, default)
         ret flag._data
     }