Update changelog

Author: vitaut

ChangeLog.md

@@ -9,6 +9,10 @@
 - Fixed deprecated `visit_format_arg` (https://github.com/fmtlib/fmt/pull/4043).
   Thanks @nebkat.
 
+- Fixed handling of a sign and improved the `std::complex` formater
+  (https://github.com/fmtlib/fmt/pull/4034,
+  https://github.com/fmtlib/fmt/pull/4050). Thanks @tesch1 and @phprus.
+
 - Removed a redundant check in the formatter for `std::expected`
   (https://github.com/fmtlib/fmt/pull/4040). Thanks @phprus.