anvil-zksync branch zkos-dev doesn't exist

### 🐛 Bug Report

#### 📝 Description

The `zkos-dev` branch doesn't exist in `anvil-zksync`. Update the README. 

#### 🔄 Reproduction Steps

See README in this repo.

> Run [anvil-zksync from github](https://github.com/matter-labs/anvil-zksync) - IMPORTANT - make sure to use zkos-dev branch.

#### 🤔 Expected Behavior

Branch exists

#### 😯 Current Behavior

Branch doesn't exist
