minor errors in JSON-LD examples

[acoburn]

There is a minor syntactical error in one JSON-LD example (a missing comma).

There is also an inconsistency in how elided JSON-LD is represented (both // ... and ... appear in the examples). This PR changes instances of // ... to ..., making the WA examples consistent with how elided JSON-LD is represented in the JSON-LD specification.

[iherman]

Marked as non-substantive for IPR from ash-nazg.