Fix appcast hash for NewNewsWire

Seems like I made a mistake with the appcast hash in  #7177.

The new (i.e. corrected) hash seems to be the same as in the pre-#7177 cask.
The new one was determined via `curl https://updates.blackpixel.com/updates\?app\=nnw | shasum -a 256`

Sorry for the trouble. :sweat_smile: