Fix final page row count in parquet-index binary

[tustvold]

Which issue does this PR close?

Closes #.

Rationale for this change

$ cargo run --bin parquet-index --features cli ../parquet-testing/data/alltypes_tiny_pages.parquet int_col
...
Page   324 at offset 0x00012ebc with length         33 and row count         16, min          0, max          9

Which agrees with

$ cargo run --bin parquet-layout --features cli ../parquet-testing/data/alltypes_tiny_pages.parquet | jq -r '.row_groups[-1].columns[4].pa
ges[-1]'
    Finished dev [unoptimized + debuginfo] target(s) in 0.12s
     Running `/home/raphael/repos/external/arrow-rs/target/debug/parquet-layout ../parquet-testing/data/alltypes_tiny_pages.parquet`
{
  "compression": null,
  "encoding": "plain_dictionary",
  "page_type": "data_page_v1",
  "compressed_bytes": 16,
  "uncompressed_bytes": 16,
  "header_bytes": 17,
  "num_values": 16
}

What changes are included in this PR?

Are there any user-facing changes?

[tustvold]

I created #3555 to track properly testing these

[ursabot]

Benchmark runs are scheduled for baseline = 19e3e8c and contender = 0ec5f72. 0ec5f72 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java