Editorial issue in invocation.startTimeUtc and invocation.endTimeUtc descriptions

[michaelcfanning]

As below the descriptions for invocation.startTimeUtc and invocation.endTimeUtc should refer to the invocation and not the run instance. This correction was likely missed when we created the notion of multiple invocations to represent a single run (moving the relevant UTC properties to the invocation type).

"description": "The Coordinated Universal Time (UTC) date and time at which the run started

@lgolding, FYI

[michaelcfanning]

We may consider updating descriptions for the invocation machine, processId and workingDirectory descriptions as well.

@michaelcfanning executableLocation also needed tweaking.