Fixed constant typo in the periodic boundary conditions

[michaeldavidjohnson]

Previously, there has been a typo throughout the code implementation in Step 12 in the periodic boundary conditions. As a quick example, If u_{i,j} was at the right boundary, then u_{i+1,j} was took to be at the left boundary i.e u_{0,j} (instead of u_{1,j}). This PR fixes those issues, although hardly changes the results.

[bugleilei]

您的来信已收到，谢谢！