fix: Use correct feature name for canbench profiling (#296)

The feature that enables specific canbench profiling of scoped code
parts is named `canbench-rs` in
[Cargo.toml](https://github.com/dfinity/stable-structures/blob/77ccf6cd06765cdd1ee6478c3865c27885ebf24e/Cargo.toml#L17)
since a while ago. It seems that some of these were missed [when the
other cases were
updated](a6cdac2).

Author: dsarlis