Add plural labels to entity types

This change record has exists for sometime ( april '16 )

https://www.drupal.org/node/2689949

It is strongly recommended that each entity type provides the following annotation keys:

* label_singular = @Translation("content item"),
* label_plural = @Translation("content items"),
* label_count = @PluralTranslation(
* singular = "@count content item",
* plural = "@count content items",
* ),

Today core has systematically gone though of all it entity types and made sure each follows this convention.

#2702683: Add plural labels to entity types

As far as the Flag module's content entites go "Flagging" has the appropriate annonation keys. Meh "flag" not so much

so this issue is a minor quick fix.