fix index in vsm_01_distributional.ipynb

[lorarjohns]

Changes w_{1} to w_{i} in the cell that reads

Now build a matrix $M$ of dimension $|V| \\times |V|$. 
Both the rows and the columns of $M$ represent words. 
Each cell $M[i, j]$ is filled with the value $d[(w_{1}, w_{j})]$.

[cgpotts]

@lorarjohns Thank you for this!