message: core: fix missing `dyn` in `Visit` docs example
diff: @@ -115,7 +115,7 @@ pub struct Iter {
  /// }
  ///
  /// impl<'a> Visit for StringVisitor<'a> {
  -/// fn record_debug(&mut self, field: &Field, value: &fmt::Debug) {
  +/// fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
  /// write!(self.string, "{} = {:?}; ", field.name(), value).unwrap();
  /// }
  /// }
  