Fix documentation for register_node pos

[CorsCodini]

The docs currently describe the pos argument as list[int, int], but the method actually expects a tuple (int, int).
The basic example from here (https://chantonic.com/NodeGraphQt/api/examples/ex_overview.html) also uses tuple.
Note: if lists were intended, the correct type would be list[int], not list[int, int].