Update AlphabetJuzzleBoard.java

Changed line 29 from "int k = i * j + j;" to "int k = i * c + j;"

In this case we have a 4x4 grid. Consider what happens on the second iteration of the loop on line 27. In this case we want the outcome on line 30 to be:

alphabetBoard[1][0] = LOWERCASE.getIndex(letters[4]);

But under the equation on line 29, we get int k = i * j + j = (1 * 0) + 0 = 0. This leaves us with: 

alphabetBoard[1][0] = LOWERCASE.getIndex(letters[0]);

Other values are similarly miscalculated. The new equation, "int k = i * c + j;" fixes this problem. I'm assuming it's what the author originally intended.