examples: update UDT parsing examples

[DylanVerstraete]

I found some discrepancy between the examples, specifically how to parse a row into a custom type:

scylla-rust-driver/examples/user-defined-type.rs

Line 51 in e6d6d3e

let (my_val,) = row?;

and the docs about UDT.

Would be nice if the examples could be updated.