refine formula, use aligned

由于上一步已经证明出了$cost$满足四边形不等式，而该不等式的左边在上式出现过，将其替换得

$f_{i,x}+f_{x+1,j}+cost_{i,j}+f_{i+1,y}+f_{y+1,j+1}+cost_{i+1,j+1}$

$\le f_{i,x}+f_{x+1,j+1}+cost_{i,j+1}+f_{i+1,y}+f_{y+1,j}+cost_{i+1,j}$

$$

\begin{aligned}

&&f_{i,\,x}+f_{x+1,\,j}+cost_{i,\,j}+f_{i+1,\,y}+f_{y+1,\,j+1}+cost_{i+1,\,j+1}\\

&\le&f_{i,\,x}+f_{x+1,\,j+1}+cost_{i,\,j+1}+f_{i+1,\,y}+f_{y+1,\,j}+cost_{i+1,\,j}\\

\end{aligned}

$$

消去公共项可得 $f_{i,j}+f_{i+1,j+1}\le f_{i+1,j}+f_{i,j+1}$