Fix cipher protocol ID type in docs

Fix cipher protocol ID type in docs

The cipher protocol ID, the return type of SSL_CIPHER_get_protocol_id,
is uint16_t and correctly described in docs to be 2 bytes, however the
function signature on the same page incorrectly pointed to it being
uint32_t, which is 4 bytes.

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/28523)