Update 2013-08-26-equality.md

This does not hold true for all immutable strings, only for those that are statically defined. Here is a simple counter example:

#import <Foundation/Foundation.h>

int main(int argc, char * argv[]) {
    @autoreleasepool {

        NSString * a = @"Hello";
        NSString * b = @"Hello";

        NSMutableString * c = [[NSMutableString alloc] init];
        [c appendString:@"Hel"];
        [c appendString:@"lo"];

        NSString * d = [[NSString alloc] initWithString:c];

        NSLog(@"a == b: %d", a == b);
        NSLog(@"a == d: %d", a == d);
        
    }
}
Merge pull request #27 from ericjohnson/master

Update 2013-08-26-equality.md