docs: correct the Panics section on Bytes::slice_ref

[hxperl]

The # Panics section on Bytes::slice_ref is wrong twice over:

/// Requires that the given `sub` slice is in fact contained within the
/// `Bytes` buffer; otherwise this function will panic.
pub fn slice_ref(&self, subset: &[u8]) -> Self {

1. It names sub; the parameter is subset, which the rest of the doc comment already uses. `sub` appears nowhere else in src/.
2. An empty subset does not panic even when it is outside the buffer — slice_ref returns early. The repo's own empty_slice_ref_catches_not_an_empty_subset test asserts exactly that.

Fixed both.