Fixed constant typo in the periodic boundary conditions #82
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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.