finite_difference_derivative with 6th Order calc coeff error value

Version: 1.85.0
file path: boost\math\differentiation\finite_difference.hpp

In function: finite_difference_derivative with 6th Order, calc step h by :
```
// Error: h^6f^(7)(x)/140 + 5|f(x)|eps/h
Real h = pow(eps / 168, static_cast(1) / static_cast(7));
```
The coeff number 168 was wrong acrooding to the comment expression below. The Number 168 seems to be calc by 140*6/5, which the correct coeff should be 140*5/6 = 350/3.
Besides, it should be eps * Coeff instead of eps / 168.
The final expression of step h should be like:
```
Real h = pow(eps * 350 / 3 , static_cast(1) / static_cast(7));
```

And, every comment about ’eps^2/3, eps^4/5, eps^6/7' were not used in the code, which also attentioned in the website document.