A typo in problem 18.2-3

in the function B-tree-find-max(). the recursive call is wrong:
```
B-TREE-FIND-MAX(x)
    if x == NIL           // T is empty
        return NIL
    else if x.leaf        // x is leaf
        return x.[x.n]    // return the maximum key of x
    else
        DISK-READ(x.c[x.n + 1])
        return B-TREE-FIND-MIN(x.c[x.n + 1])
```