Use the modern term Unit ID

[sgryphon]

The old name is used in the API and documentation for identifying the Unit Identifier.

See the MODBUS TCP/IP guide (Modbus_Messaging_Implementation_Guide_V1_0b.pdf) at https://modbus.org/specs.php

In the current documentation, the fourth parameter of the header is called Unit Identifier.

Fix: Update the name to use the modern term.