doc: replace hardcoded "my_program" with an actual name of the executable

[eugenesvk]

So that it's easier to copy&paste and match the defaults to be able to tweak them

Though I couldn't quickly find how the default is generated and where the binary name comes from there to use that mechanism

(left in "my_program" in tests)

[pacak]

It would improve the documentation, but I don't think this is going to work as is. All the code in docs2 gets compiled so documentation can contain the actual help output or error message that the example would produce, but it gets compiled as a library to keep compilation time low. One way to fix it would be setting the env variable on compile time for docs2 crate with a build script... Not sure it's worth doing it - whole crate is likely going away in the next iteration.

[eugenesvk]

Ah, ok, nevermind then, I'll replace it with a comment then and ignore docs2