[tutorial] be consistent in naming annotations

[chalin]

Migrating this issue from naomiblack/angulardart.org#16.

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.

[chalin]

Now that the html files have been reformatted I will port some previous edits done relative to this issue.