Remove whitespace in operator"" _a

[danakj]

Clang 18 warns about this whitespace.

third_party/fmt/include/fmt/format.h:4417:27: warning: identifier '_a' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
 4417 | constexpr auto operator"" _a(const char* s, size_t) -> detail::udl_arg<char> {
      |                ~~~~~~~~~~~^~
      |                operator""_a
1 warning generated.

Resolves #3607

[vitaut]

Thanks for the PR but it looks like it is causing an error in gcc.

[danakj]

Hmm, yes. I see that the syntax has changed and both are correct in different places, and the other is rejected there. I wonder why clang 18 is ending up on this branch for me.