This keeps the examples in line with our actual codebase.
Also while I'm here revise it to explain the actual motivation for our
use of `env`, and to correct some subtle details -- it's actually the
kernel that interprets the shebang (as visible in e.g. a `strace` log),
not the shell, and when the program is executed as `./my_program.py`
the exact name including `./` is passed to the interpreter.