Resolved bug in kMaxValue in SerializationTest for toml_fuzzer

[capuanob]

What does this change do?

Fixes a super simple logic msitake in the harness, where the kMaxValue was set to 1 less than the true max- preventing one branch of the switch from ever being exercised.

Is it related to an exisiting bug report or feature request? No

Pre-merge checklist

• [X] 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
• [X] I've rebuilt and run the tests with at least one of:
  • [X] Clang 8 or higher
  • [ ] GCC 8 or higher
  • [ ] MSVC 19.20 (Visual Studio 2019) or higher
• [X] I've added my name to the list of contributors in README.md

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[marzer]

Thanks :)