Fix more OCSP_resp_get0_signer() nits

Fix a typo for "retrieve" and some indentation.

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>

(cherry picked from commit eb48052e)

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4723)