Remove space in custom string literal

[MichaelChirico]

I may lack sufficient context for this to be a useful PR, so please feel free to close if this is not welcome.

For no particular reason I saw an analogous PR to another R package repo in my GitHub home page:

marzer/tomlplusplus#263

I searched the main R package repo for other similarly-affected packages, which includes {clustermq}, which bundles this repo as a submodule:

https://github.com/mschubert/clustermq/tree/master/src

Here's the documentation I could find about why this matters:

https://en.cppreference.com/w/cpp/language/user_literal

// error: all names that begin with underscore followed by uppercase
// letter are reserved (NOTE: a space between "" and _).
double operator"" _Z(long double);

[coveralls]

Pull Request Test Coverage Report for Build 13804398584

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.368%

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Totals
Change from base Build 13540525397:	0.0%
Covered Lines:	849
Relevant Lines:	983

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💛 - Coveralls

[gummif]

We probably need to modify the clang-format file/update clang-format so this doesn't get reverted, but this is fine for now.