Fix comment that does not match the code

Description

Fix the comment that does not match the code in src/sponge/absorb.rs line 68: "Append the list to dest." -> "Return the list as Vec."

As the function batch_to_sponge_field_elements_as_vec returns Vec, the changed comment is more appropriate.

[x] Targeted PR against correct branch (main)
[x] Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
[ ] Wrote unit tests (comment line doesn't need a unit test)
[x] Updated relevant documentation in the code
[ ] Added a relevant changelog entry to the Pending section in CHANGELOG.md (It doesn't need a change log, because it is not a significant change)
[x] Re-reviewed Files changed in the Github PR explorer