Update RELEASING.md

We use a trick to be able to translate English strings on Transifex: We
overwrite `en.json` with `en_US.json`.

RELEASING.md

@@ -6,6 +6,7 @@ with deprecations. Remove them if possible.
 Update translations (https://github.com/transifex/transifex-client/):
 
     $ tx pull --all
+    $ mv public/i18n/en_US.json public/i18n/en.json
     $ git diff public/i18n/
     $ git add public/i18n/
     $ git commit -m 'Update translation strings from Transifex'

public/i18n/en.json

@@ -218,8 +218,7 @@
         "SENT": "The message was delivered to the Threema server",
         "DELIVERED": "The message was delivered to the recipient's device",
         "READ": "The message was read by the recipient",
-        "FAILED": "The message could not be sent",
-        "UNKNOWN": ""
+        "FAILED": "The message could not be sent"
     },
     "messageTypes": {
         "AUDIO_MESSAGE": "Audio Message",