Fix doc test

/// use openssl::crypto::hash::Type;

/// use openssl::crypto::hmac::HMAC;

/// let key = b"Jefe";

/// let data = [b"what do ya ", b"want for nothing?"];

/// let data: &[&[u8]] = &[b"what do ya ", b"want for nothing?"];

/// let spec = b"\x75\x0c\x78\x3e\x6a\xb0\xb5\x03\xea\xa8\x6e\x31\x0a\x5d\xb7\x38";

/// let mut h = HMAC::new(Type::MD5, &*key);

/// h.write_all(data[0]);