doc/man3/OSSL_PARAM_TYPE.pod: modify Example 2 to allow unspecified params

A parameter requestor is never obligated to ask for all available
parameters on an object.  Unfortunately, Example 2 showed a code
pattern that introduced such an obligation, and therefore needed a
small adjustment.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8523)