Change the parameters of `derived_descriptor`

[sanket1729]

The parameter for secp should go first, idx should go second.

See:

rust-miniscript/src/descriptor/mod.rs

Line 591 in 6a89bd0

pub fn derived_descriptor<C: secp256k1::Verification>(

Update the xpub_descriptors.rs example to use this API instead of derive

[Eunoia1729]

Hi @sanket1729 ! I'd like to work on this