Remove space from user defined string literal

[hpkfft]

The Intel icpx 2025.2.0 compiler, which is based on clang, warns that identifier '_s' preceded by whitespace in a literal operator declaration is deprecated.

That seems to be correct. https://en.cppreference.com/w/cpp/language/user_literal.html documents

user-defined-string-literal the character sequence "" followed, without a space, by the character sequence that becomes the ud-suffix

Note that nanobind's definition of operator""_a in nb_attr.h does not have whitespace, so this PR is consistent.