Fix trivial typo in EVP_DigestVerifyInit doc

(cherry picked from commit b3883f77df33989b0d4298ca9a21d8595dd9a8c9)

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8319)