Correct descriptions of failed examples in valid language tag rule

[sabernhardt]

The first two failed examples do not have a known primary language tag, and the third example could say "three-letter code."

Need for Call for Review:
This will not require a Call for Review: editorial changes (examples section)

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[CLAassistant]

All committers have signed the CLA.

[Jym77]

Well spotted 👍 A bit of polishing.

[giacomo-petri]

Approving with @Jym77's changes.

[carlosapaduarte]

Committing the changes proposed by @Jym77 to try to move this one forward :-)

[carlosapaduarte]

Merging.
Editorial changes, no call for review required.