Change name of MMR function get_peak_index_and_height

[Sword-Smith]

This function returns the node index of a peak, not index into the MMR accumulator's peak list. Luckily it's a private function in twenty-first/src/util_types/mmr/mmr_membership_proof.rs, but it should still be changed, as we use the term "peak index" to refer to the index into the peaks list.