Fix README markdown

This PR contains a cosmetic README fix.

Additionally:

There is a line inconsistent with the code in the README:

```
All protocol changes are retroactive on testnet.
```

This is not true as per https://github.com/CounterpartyXCP/counterpartyd/blob/develop/lib/blocks.py#L1182-L1186

I suggest removing or changing this line to read:

```
Testnet parsing begins on block 303000.
```

Further discussion https://github.com/CounterpartyXCP/counterpartyd/pull/333#issuecomment-58943235