README: Fix invalid escape sequence in string

The README contains an example of converting a string literal to a
regex, but the string literal contains an invalid escape sequence: "\\."

Of course, this is a legal regex pattern, but in order to silence the
warning, use "\\\\." instead. The pattern matches as it does without the
extra escape.

---

The following displays a warning ("invalid escape sequence"):

```objc
@"^http://(.*?)\.crunchtime-labs\.com/v1/estimates\.csv".regex
```

Whereas this does not:

```objc
@"^http://(.*?)\\.crunchtime-labs\\.com/v1/estimates\\.csv".regex
```

Both match the same patterns. That is, either one will match against `http://api.crunchtime-labs.com/v1/estimates.csv`.

Based on the README I first assumed that it was a Nocilla issue, but in fact the regex in the README just needs a little updating.