doc: fix spelling of TYPE_get_ex_new_index

The generated macros are TYPE_get_ex_new_index() (to match
CRYPTO_get_ex_new_index()), not TYPE_get_new_ex_index(), even though
the latter spelling seems more natural.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10943)
