Fix to sample code for href in ReactPropTypes

Fix to 2 problems in the sample code for ReactPropTypes:
- Condition was inverted (error should be thrown when the previous condition is "not" met).
- Missing null check (the href propType is described as optional in the comment but a null in the prop would have failed the check)
Ahh yes, you're right. Sorry, this looks good to me. One small thing: can you wrap the line like this so it fits in 80 chars?

```
 *        if (propValue != null && typeof propValue !== 'string' &&
 *            !(propValue instanceof URI)) {
```

Thanks!