random: update comment from copy_to_user() -> copy_to_iter()

commit 63b8ea5e upstream.

This comment wasn't updated when we moved from read() to read_iter(), so
this patch makes the trivial fix.

Fixes: 1b388e77 ("random: convert to using fops->read_iter()")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>