doc: Fix missing cmake -B option in fuzzing quickstart guide #34876

cmake --build assumes our build directory is build_fuzz, but cmake didn't use -B build_fuzz, so simply copying and pasting the commands didn't work.

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I don't understand this. The preset literally has it set to that:

      "binaryDir": "${sourceDir}/build_fuzz",

What is the output before and after the changes here?

cmake --build assumes our build directory is build_fuzz, but cmake didn't use -B build_fuzz...

It does:https://github.com/bitcoin/bitcoin/blob/6e295d8ad5fff4dc07519f570e9a04ac57ecb639/CMakePresets.json#L41

Oh, I just checked my bash history. I did not actually copy and paste the commands. :sweat_smile:

I manually edited my cmake command for clean rebuilds to include --preset=libfuzzer and --build build_fuzz instead of --build build:

$ rm -rf build && cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=1 --preset=libfuzzer -B build && cmake --build build_fuzz -j8

This obviously did not create build_fuzz, but build.

When I run cmake --preset=libfuzzer, like in the documentation, it does create build_fuzz.

Sorry! I was too quick to assume inconsistent documentation and push a fix. Should have looked into --preset first.

I guess I'm too used to inconsistent documentation (in other projects) haha :sweat_smile: