Fix the URL in the README

Should be `jamiepine/voicebox`

```
Quick Start
With Makefile (Unix/macOS/Linux):

# Clone the repo
git clone https://github.com/voicebox-sh/voicebox.git
cd voicebox

# Setup everything
make setup

# Start development
make dev
Manual setup (all platforms):

# Clone the repo
git clone https://github.com/voicebox-sh/voicebox.git
```