doc: separate unrelated info about child_process.exec()

“Never pass unsanitized user input to this function” is followed by a
code example with `bad_file`, but they aren’t related. Avoid confusion
by moving the example and giving `bad_file` a more specific name.

PR-URL: https://github.com/nodejs/node/pull/21516


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>