Fix warning: identifier '_a' preceded by whitespace in a literal operator declaration is deprecated

[phprus]

Fix error (clang-17):

In file included from /home/runner/work/fmt/fmt/test/printf-test.cc:14:
/home/runner/work/fmt/fmt/include/fmt/xchar.h:73:47: error: identifier '_a' preceded by whitespace in a literal operator declaration is deprecated [-Werror,-Wdeprecated-literal-operator]
   73 | constexpr detail::udl_arg<wchar_t> operator"" _a(const wchar_t* s, size_t) {
      |                                    ~~~~~~~~~~~^~
      |                                    operator""_a

[vitaut]

This is a warning promoted to an error with -Werror but otherwise LGTM. Thanks.