Updates string is "n Themes Updates" but "n Plugin Updates"

[mitchoyoshitaka]

The UI string which presents the number of different kinds of updates available (code in a few different places, hopefully to be consolidated by #17694) has the plural string '%d Themes Updates' even though for plugins it's '%d Plugin Updates'. Should the "theme" and "plugin" both be plural? Both be singular?

[nacin]

In [18157]:

s/Themes Updates/Theme Updates/. props mitchoyoshitaka. fixes #17701.