src/fmt/format.h: Clang-20 deprecated code fix
Fixes this deprecation warning for Clang-20:
/home/muxianesty/readcells/src/fmt/format.h:4418:27: warning: identifier '_a' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
4418 | constexpr auto operator"" _a(const char* s, size_t) -> detail::udl_arg<char> {
| ~~~~~~~~~~~^~
| operator""_a