Make Turtle examples consistent with other RDF 1.2 documents

[niklasl]

This is a tracking issue to ensure that the Turtle examples follow the conventions decided for all RDF 1.2 documents.

Specifically, we have agreed to use PREFIX instead of @prefix.

It needs to be decided whether to introduce prefixes whenever they are used in each example (as the primer currently does it, albeit with some inconsistencies), or to keep each example entirely self-contained (which may become "wordy" for some short examples making use of multiple prefixes previously introduced). Alternatively, we may declare an initial "prelude" of all prefixes that may appear in subsequent examples.