add explicit mt:: and std:: to avoid ambiguous call when both std::format_to and mt::format_to are present (#3259)

README.md

@@ -303,7 +303,7 @@ struct fmt::formatter<my_type> : fmt::formatter<std::string>
 {
     auto format(my_type my, format_context &ctx) const -> decltype(ctx.out())
     {
-        return format_to(ctx.out(), "[my_type i={}]", my.i);
+        return fmt::format_to(ctx.out(), "[my_type i={}]", my.i);
     }
 };