Fix typo in afl_driver.cpp

[eltociear]

Fixed typo.

occured -> occurred

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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.

[coveralls]

coverage: 100.0%. remained the same when pulling 151b59846cb88d5aae8ae14c3f7e4b5366d51e9e on eltociear:develop-1 into 5d2754306d67d1e654a1a34e1d2e74439a9d53b3 on nlohmann:develop.