Fixes AES benchmarks

Summary: It fixes a typo in AES256 benchmarks.

Test Plan:

ninja check
ninja bench-bitcoin

Reviewers: Fabien, #bitcoin_abc, deadalnix, nakihito, jasonbcox

Reviewed By: #bitcoin_abc, nakihito, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4749