Fix toString()

src/espchrono.cpp

@@ -262,7 +262,7 @@ std::expected<std::chrono::seconds, std::string> parseDaypoint(std::string_view
 
 std::string toString(const DateTime &dateTime)
 {
-    return fmt::format("{:04}-{:02}-{:02}T{:02}:{:02}:{:02}.{:03}",
+    return fmt::format("{:04}-{:02}-{:02}T{:02}:{:02}:{:02}.{:03}Z",
                        int{dateTime.date.year()}, unsigned{dateTime.date.month()}, unsigned{dateTime.date.day()},
                        dateTime.hour, dateTime.minute, dateTime.second, dateTime.millisecond);
 }