Address clang++20 nag about 'whitespace in literal operator'

[eddelbuettel]

As discussed in #261, clang++-20 now nags about a whitespace after the """ in operator"". A simple and sufficient change is to remove the whitespace. I have done so in RcppTOM and the CRAN repository for R which is already running some checks with clang++-20 no longer tags the package.

I carried the same change over here.

Note that the embedded catch and nlohmann::json have the same issue in operator "" (the initial whitespace between operator and "" is apparently permitted) in a handful of places but as this is only used for testing I left it alone.

Pre-merge checklist

• I've read [CONTRIBUTING.md]
• I've rebased my changes against the current HEAD of origin/master (if necessary)
• I've added new test cases to verify my change
• I've regenerated toml.hpp ([how-to])
• I've updated any affected documentation
• I've rebuilt and run the tests with at least one of:
  • Clang 8 or higher
  • GCC 8 or higher
  • MSVC 19.20 (Visual Studio 2019) or higher
• I've added my name to the list of contributors in README.md

[marzer]

Thanks 🎉