change id to instancetype in initWithCoder

Hi! It's me again.
Today when i want to debug my app, i print:
po myObject

and get error:
error: instance method 'initWithCoder:' has incompatible result types in different translation units ('id' vs. 'NSObject *')
note: instance method 'initWithCoder:' also declared here
error: 1 errors parsing expression

And i understand your mistake in initWitCoder in NSObject+RMArchivable and fix.