Update docs ref deprecated type `DerivationDriver`

[emhane]

Update docs referencing deprecated type DerivationDriver

kona/docs/docs/pages/sdk/proof/custom-backend.mdx

Lines 87 to 88 in 4733473

	From here, a custom derivation driver is needed to produce the desired execution payload(s). An example of this for
	`kona-client` can be found in the [DerivationDriver](https://github.com/op-rs/kona/blob/main/bin/client/src/l1/driver.rs#L77).

https://github.com/op-rs/kona/actions/runs/17097895931/job/48486594111?pr=2689#step:3:813