Finish cleaning-up annotation class names in chapters 3 to 7.

[chalin]

This issue was originally reported here: angular.dart.tutorial/issues/54: the issue text reads:

In some places annotations are written as @annotation and others the @ is missing. In most cases the annotation names are show as plain text, but we put, e.g., directives in back ticks [i.e. format it as code], I think that annotations should be in back ticks too.

I have cleaned-up Chapter 2 via 7031a3c, but the other chapters should be cleaned up too.