Fix two validation errors in PaNET_metadata.ttl

[paulmillar]

Motivation:

The Apache Jena tool riot discovered two problems with the current content of PaNET_metadata.ttl:

• A multi-line text literal is not allowed for values contained within single quotes.
• The set of assertions are not terminated with a dot.

Modification:

Switch string literal to use long-quote.

Add missing terminating dot.

Result:

The PaNET_metadata.ttl file is now valid Turtle.

Closes: #144