Remove last update line from project root README

[j-d-b]

This PR removes the following line from the project root README:

Last updated 9/3/2020 - WZDx Specification v3.0

The change was made for the following reasons:

• The line is unnecessary—one can readily see via the GitHub web UI when the repo and every specific file was last updated
• The line has not been updated consistently when changes were made
• It is not clear what updates would warrant updating the line
• One can check both the Releases section of the right pane on GitHub and the RELEASES.md file and the most current release

[j-d-b]

@chuehlien thank you for the quick review and for the great work on GitHub recently! We are both using the verb-first commit messages and the review feature and the repo looks really clean.