doc: correct the build instructions for the example #202

I think we prefer

cmake -B build
make -C build mpexample
build/example/mpexample

I tried the cmake -B build approach as well, but this generates the warning kj/filesystem-disk-unix.c++:1740: warning: PWD environment variable doesn't match current directory; pwd = <PATH_To_LOCAL_REPO>/libmultiprocess . This warning occurs because there's a mismatch between:

• The PWD environment variable and
• The actual current directory.

It does not affect the output, but I prefer to avoid the warning entirely.

The rest of this project follows the mkdir build, cd build pattern, so seems fine to stick to that.

The following does build for me on macOS:

cmake -B build
cmake --build build --target mpexample

But then the example itself won't work:

% build/example/mpexample
execvp failed: No such file or directory
libc++abi: terminating due to uncaught exception of type std::runtime_error: IPC client method call interrupted by disconnect.
zsh: abort      build/example/mpexample

In commit "doc: correct the build instructions for example" (b6d3dc44194c62e6baab8315266c0c745f61036b)

Hmm, this all seems ok, but the PWD thing seems like something that could be followed up later. Would want to understand where exactly the warning is coming from and avoid it, maybe by unsetting PWD. @Sjors I think probably the reason you saw that error was because you built the mpexample target not mpexamples. mpexample is just the main example binary but it depends on mpcalculator and mpprinter. mpexamples is a target that builds all 3 binaries. This could also deserve a followup to be less confusing.

You're right, --target mpexamples (with an "s") works:

% build/example/mpexample
Enter the equation, or "exit" to quit: How much is the fish?
mpprinter: Wow How much is the fish?, that's a tough one.