vinyl: fix comment to vy_read_iterator_cmp_stmt()

vy_read_iterator_cmp_stmt() uses tuple_compare_with_key() under the hood
which follows next convention:
(a) it returns 0 if key_fields of tuple equal to given key;
(b) < 0 if key_fields less than given key;
(c) > 0 otherwise.
However, comment to vy_read_iterator_cmp_stmt() says that:
(a) it returns -1 if first statement precedes second one;
(b) 0 if they at the same position;
(c) +1 if first statement supersedes second one.
so, since there's no "normalization" of return code from
tuple_compare_with_key() it would be correct to say that
vy_read_iterator_cmp_stmt() returns:
(a) it returns arbitrary integer value < 0 if first statement precedes
second one;
(b) 0 if they at the same position;
(c) arbitrary integer value > 0 if first statement supersedes second one.