Update README.md

README.md

@@ -293,7 +293,7 @@ void multi_sink_example2()
 template<>
 struct fmt::formatter<my_type> : fmt::formatter<std::string>
 {
-    auto format(my_type my, format_context &ctx) -> decltype(ctx.out())
+    auto format(my_type my, format_context &ctx) const -> decltype(ctx.out())
     {
         return format_to(ctx.out(), "[my_type i={}]", my.i);
     }