Fix issue 21444: bad string concat in static assert message

[berni44]

trivial fix

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21444	trivial	bad string concat in static assert message

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7746"