const var NotificationName should not use static

[KeithMorning]

In the section of https://github.com/objc-zen/objc-zen-book#constants, we should not use static for NotificationName, or we can' t extern it for other code. For example:

//a.h
extern static NSString *const TestConstNotificationName;

//a.m
static NSString *const TestConstNotificationName = @"TestConstNotificationName";

This won't pass compile.

On other hand, a constant will be init in the global memory area. So it doesn't need a static key word.