docs: update 8.5.0 changelog

The original changelog included incorrect information regarding
the new perf_hooks api.

refs: https://github.com/nodejs/node/pull/15308#issuecomment-328874385

PR-URL: https://github.com/nodejs/node/pull/15384


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>