Part 2: Editorial changes involving "instance of time"

As an editorial change I suggest changing Section 7.1.1 of Part 2 in the following ways: 

* In the phrase "a reasonable approximation of the current instance in Coordinated Universal Time", change "instance" to "instant".
* In the sentence "An HTTP-date value represents time as an instance of Coordinated Universal Time (UTC).", change "instance of" to "instant in".

--Peter