docs: Fix variable names in Indexed Merkle Tree range check documententation.

[just-mitch]

In the documentation of non-membership proof of the indexed merkle tree, the range check for when the low nullifier is not at the very end refers to the low_nullifier where it ought to refer to new_value.

[ludamad]

Hi. We've recently decided on a policy that we will not be merging trivial PRs unless someone has done a significant contribution already. The recent usage of Github commits as a metric mean we have to manage this a bit. We will redo this change and thank you for the contribution.

It will be merged here #3556