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)
Error should be thrown only when all these are false. So is not null (because nulls are ok), is not a string (because strings are ok), and is not an instance of URI (becase URIs are ok).