Update provide-inject.md (vuejs#2317)

Author: davidkrijgsman

src/guide/components/provide-inject.md

@@ -208,9 +208,11 @@ const value = inject('message', 'default value')
 In some cases, the default value may need to be created by calling a function or instantiating a new class. To avoid unnecessary computation or side effects in case the optional value is not used, we can use a factory function for creating the default value:
 
 ```js
-const value = inject('key', () => new ExpensiveClass())
+const value = inject('key', () => new ExpensiveClass(), true)
 ```
 
+The third parameter indicates the default value should be treated as a factory function.
+
 </div>
 
 <div class="options-api">