command needs double quotes to work

update usage instructions in README.md to include double quotes:
## Usage

Be on the same wifi as your chromecast and do

```
peercast magnet:?xt=urn:btih:99feae0a05c6a5dd9af939ffce5ca9b0d16f31b0
```

should be:

```
peercast "magnet:?xt=urn:btih:99feae0a05c6a5dd9af939ffce5ca9b0d16f31b0"
```
