fix: correct array indexing in commitment functions to prevent out-of-bounds access

[VolodymyrBg]

Fix array index calculation in three commitment functions that were causing
potential out-of-bounds access when storing the last element:

• hash_salt_dg1_private_nullifier: correct index for private_nullifier
• calculate_private_nullifier: correct index for sod_sig
• hash_salt_country_signed_attr_dg1_e_content_private_nullifier: correct index for private_nullifier

The issue was that the index calculation was using the array size instead of
size - 1 for the last element position. This could lead to runtime errors
with certain input sizes.

Also fix typo in compute_merkle_root comment (removed duplicate "the").

[michaelelliot]

@VolodymyrBg Can you please pull the latest changes into your branch? It should include the GitHub Actions CI workflow.

[VolodymyrBg]

@VolodymyrBg Can you please pull the latest changes into your branch? It should include the GitHub Actions CI workflow.

Done

[michaelelliot]

@VolodymyrBg Looks like the tests failed on your branch.