Fix grammar

_Question: What is the value of window.foo?_
```javascript
( window.foo || ( window.foo = "bar" ) );
```

**Answer: "bar"** _(only if window.foo was falsey otherwise it will retain its value)_
is better as
**Answer: "bar"** _(only if `window.foo` is truthy, does it retain its value)_

The new explanation is both less wordy and less confusing.