Explicitly state the bootstrap command

@saurik just tripped up on this at JailbreakCon. He read:

```
/opt/theos is not writable. Please run ./bin/bootstrap.sh substrate manually, with privileges.
```

and ran `./bin/bootstrap.sh`, without the substrate argument. This fixes that:

```
/opt/theos is not writable. Please run `./bin/bootstrap.sh substrate` manually, with privileges.
```