doc: fix Block interface parameters order

The Effective Go document references the crypto/cipher Block
interface, but the parameters' names are swapped. This change
alignes them to the crypto definition, to avoid confusion.

Fixes #33432

Change-Id: I8b9aac4dc6af3eec968bbc8f3ee5366b99016fcc
Reviewed-on: https://go-review.googlesource.com/c/go/+/188797


Reviewed-by: Ian Lance Taylor <iant@golang.org>