Fix a typo in example

Author: vitaut

include/fmt/color.h

@@ -634,7 +634,7 @@ struct formatter<detail::styled_arg<T>, Char> : formatter<T, Char> {
 
   **Example**::
 
-    fmt::print("Elapsed time: {s:.2f} seconds",
+    fmt::print("Elapsed time: {0:.2f} seconds",
                fmt::styled(1.23, fmt::fg(fmt::color::green) |
                                  fmt::bg(fmt::color::blue)));
   \endrst