Fix to Enable Display Features - Google Analytics iOS SDK

In the official Google Analytics iOS SDK documentation, this function call will enable Display Features in your app:

id tracker = [[GAI sharedInstance] defaultTracker]; // Enable Advertising Features. 
[tracker set:allowIDFACollection value:@YES]; 

Sadly, this causes an error, as allowIDFACollection is an invalid key. The real way to enable display features is to call this method:

[[GAI sharedInstance].defaultTracker setAllowIDFACollection:YES]; 

Save yourself a few minutes of frustration. Build and be happy.