i18n: Follow stream-channel rename in our one translation, too

Before launch, in order to manage contributions from many people
for translations into many languages, we'll wire things up to
an appropriate platform: #276.  For now, though, we have just this
one partial translation into Japanese, as an end-to-end demo of the
i18n setup within the app, and we maintain it directly in the repo.
Update that translation to reflect this rename.

Author: gnprice

assets/l10n/app_ja.arb

@@ -5,7 +5,7 @@
   "cameraAccessDeniedTitle": "権限が必要です",
   "cameraAccessDeniedMessage": "画像をアップロードするには、「設定」で Zulip に追加の権限を許可してください。",
   "cameraAccessDeniedButtonText": "設定を開く",
-  "subscribedToNStreams": "{num, plural, other{{num}つのストリームをフォローしています}}",
+  "subscribedToNStreams": "{num, plural, other{{num}つのチャンネルをフォローしています}}",
   "userRoleOwner": "オーナー",
   "userRoleAdministrator": "管理者",
   "userRoleModerator": "モデレータ",