Minor: Fix path in format command in CONTRIBUTING.md

[etseidl]

Which issue does this PR close?

N/A

Rationale for this change

The instructions for running cargo fmt on the parquet crate will scan too many files.

What changes are included in this PR?

Change instructions so only the parquet directory will be traversed.

Are there any user-facing changes?

No, documentation change only.

[alamb]

Looks good to me -- thank you @etseidl

[alamb]

I double checked that the corresponding CI job doesn't need to be changed too

arrow-rs/.github/workflows/rust.yml

Lines 108 to 109 in 25e1969

	working-directory: parquet
	run: cargo fmt -p parquet -- --check --config skip_children=true `find . -name "*.rs" \! -name format.rs`

I believe it does not need a change as it already has a working-directory set to parquet