Update instructions about verifying contracts

[deuszx]

Instruction about how to verify the contract in the README use some very specific paths, like roccoco etc., but it's not necessary when verifying. The following was enough to verify locally:

1. Correct structure of the contracts/package locally:

build
├── package.zip
└── pristine.wasm

2. Running the following command:

docker run --rm -v ./build:/build ink-verifier:develop

If instructions can be simpler and not roccoco-specific (or any other chain) we should try to simplify it.