Fix doc-nits in doc/man3/DEFINE_STACK_OF.pod

<compar> to <compare> to match the var name in function prototype

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4559)