fix: correct the get_parkinson_vol spelling in the public API

[Sean-Koval]

Replaces #23, which GitHub auto-closed when its base branch (chore/cargo-fmt-pyopenquant, now merged as #20) was deleted. Same commit, retargeted to main.

Breaking change, made on the owner's instruction.

Parkinson was spelled parksinson in both the Rust function and the Python binding. Because
#[pyfunction(name = "get_parksinson_vol")] is the public Python API, correcting it breaks
downstream callers — which is why it was raised before being fixed.

Renamed:

• openquant::util::volatility::get_parksinson_vol → get_parkinson_vol
• the pyopenquant wrapper fn, its #[pyfunction(name = …)], and its module registration
• crates/openquant/tests/volatility_features.rs

One thing deliberately left misspelled

The doc comment still names mlfinlab's get_parksinson_vol, because that misspelling is
upstream's and is where ours came from. It now says so rather than propagating it silently.

Verification

cargo fmt -- --check → 0 · cargo test --package openquant --test volatility_features → 0 (1 passed) ·
cargo check --package pyopenquant → 0

Follow-up

moduleDocs.ts and the generated modules/util-volatility.md still carry the old name in four
places. Not fixed here — this branch carries the pre-#18 page generator, so regenerating from it
would revert the honest-status work. Tracked; #21's check:examples gate will fail loudly until it
is done, which is correct.