Fix OSSL_FUNC_keymgmt_load declaration in man7/provider-keymgmt

Fix OSSL_FUNC_keymgmt_load declaration in man7/provider-keymgmt

OSSL_FUNC_keymgmt_load prototype declared in man7 does not match
the actual OSSL_FUNC_keymgmt_load prototype declared in
include/openssl/core_dispatch.h. This commit fixes the prototype
in man7.

CLA: trivial

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/27189)