timing_load_creds: Fix typos in the timersub macro

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19865)

(cherry picked from commit f1b104953af8e3a82f7c5ee33f0403fc067c8516)
