Fixed incorrect flow type output

For an optional property, the property can either be undefined, otherwise must be any value that is not null. The example was showing  the incorrect output of a setting the value to null.

flow.md

@@ -133,7 +133,7 @@ type Album = {
 ```js
 const a: Album = { } // ✓ OK
 a.name = 'Blue'      // ✓ OK
-a.name = null        // ✓ OK
+a.name = null        // ✓ Error
 a.name = undefined   // ✓ OK
 ```