examples: fix index-out-of-bounds panic in bucket_add example

[aztecEagle22]

What does this PR do?

Fixes an off-by-one error that causes the bucket_add example to panic before it even starts:

thread 'main' panicked at sdk/examples/bucket_add.rs:29:23:
index out of bounds: the len is 2 but the index is 2

THe code checks that args.len() is 2, because the key is in args[1] not args[2] since index 2 is out of bounds:

    if args.len() != 2 {
        return Err(anyhow!("Usage: [private key]"));
    }

    let pk_kex = &args[2];

The example now runs as intended:

$ cargo run --example bucket_add -- <HEX_PRIVATE_KEY>
Created new bucket …

Changes

• sdk/examples/bucket_add.rs
  • Replace let pk_kex = &args[2]; with let pk_hex = &args[1];