Update MPGNotification.m

changing the code to silence Xcode warning 
Sending 'const NSString *__strong' to parameter of type 'NSString *' discards qualifiers

on lines 

        self.titleLabel.font = [UIFont fontWithName:kTitleFontName size:kTitleFontSize];

and

        self.subtitleLabel.font = [UIFont fontWithName:kSubtitleFontName size:kSubtitleFontSize];