Update documentation after the removal of assign_pgn_request_protocol_to_internal_control_function

[martonmiklos]

Here the docs mentions the ParameterGroupNumberRequestProtocol::assign_pgn_request_protocol_to_internal_control_function

AgIsoStack-plus-plus/sphinx/source/Tutorials/PGN Requests.rst

Line 49 in c9c3ea2

isobus::ParameterGroupNumberRequestProtocol::assign_pgn_request_protocol_to_internal_control_function(<your internal control function goes here>);

however in ae7ca76 the static PGN request handling was removed (including this function).

If I understand correctly this assignment is no longer needed, but clarification would be needed on this one.