In order to squash the spam here a bit I will answer....
I know from practicing another art how difficult it can be for anyone to have proper let alone 'straight' form. I see new people who can't get shoulders aligned straight, and others who may lean back or even towards the shot. So how does having a 'straight' mind relate?
If straight means direct or focused in this sense, then possibly the ability to focus on what you are doing will straighten your body.
If straight means or relates to the spirit, then having such a mindset would perhaps make the body strong and allow it to straighten.
If straight means having a just way about the individual, then perhaps this reflects in the shooting form.
Of course I tied this to shooting and it need not be applied simply to this. The connotations can go beyond our daily practice of archery. It has been said after all that the mind is the most powerful organ in the body and what is in our mind can greatly influence the body.
If the mind is 'straight' or lets say properly aligned or upright then so will the body. The saying reminds us that we must sharpen our minds perhaps even more than our bodies.