unix::ExitStatusExt: Correct reference to _exit system call

As discussed here
 https://github.com/rust-lang/rust/pull/88300#issuecomment-936085371

exit is (conventionally) a library function, with _exit being the
actual system call.

I have checked the other references and they say "if the process
terminated by calling `exti`".  I think despite the slight
imprecision (strictly, it should read iff ... `_exit`), this is
clearer.  Anyone who knows about the distinction between `exit` and
`_exit` will not be confused.

`_exit` is the correct traditional name for the system call, despite
Linux calling it `exit_group` or `exit`:
  https://www.freebsd.org/cgi/man.cgi?query=_exit&sektion=2&n=1

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/os/unix/process.rs

@@ -207,7 +207,7 @@ fn arg0<S>(&mut self, arg: S) -> &mut process::Command
 /// [`ExitStatusError`](process::ExitStatusError).
 ///
 /// On Unix, `ExitStatus` **does not necessarily represent an exit status**, as
-/// passed to the `exit` system call or returned by
+/// passed to the `_exit` system call or returned by
 /// [`ExitStatus::code()`](crate::process::ExitStatus::code).  It represents **any wait status**
 /// as returned by one of the `wait` family of system
 /// calls.