Fix typo in FuzzerMerge.cpp

[eltociear]

Fixed typo.

untill -> until

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Pull request checklist

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• [x] Changes are described in the pull request, or an existing issue is referenced.
• [ ] The test suite compiles and runs without error.
• [ ] Code coverage is 100%. Test cases can be added by editing the test suite.
• [ ] The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[nlohmann]

This is a thirdparty file. Please fix upstream.

[coveralls]

coverage: 100.0%. remained the same when pulling d38974e67208fd9e18d464cdd5e5ba8f65669751 on eltociear:patch-2 into 8c391e04fe4195d8be862c97f38cfe10e2a3472e on nlohmann:develop.